fix: monster ability queries no longer have side effects (#2716)

## Problem
`GetMonsterAbilities()` answers three questions through one virtual: which abilities a creature has (`GetAbility`, `AOS.cs` reflect/barrier, bard fire-breath difficulty), which abilities are enabled right now, and where cooldown state must be cleaned up (`InvalidateNextAbilityTriggers` on death and delete). Every caller assumes it is a pure, stable list. About 70 creatures return a static array; only LordOaks computed it:

```
Mobile.Delete()
 → BaseCreature.OnDelete() → CreatureDeletedEvent
 → MonsterAbility.InvalidateNextAbilityTriggers()
 → LordOaks.GetMonsterAbilities() → CheckQueen() → new Silvani().MoveToWorld(...)
```

- Deleting a Lord Oaks nobody had fought (champion reset, `DeleteCreatures`, GM delete) spawned an orphaned Silvani where he stood.
- The spell damage scalar (`AlterDamageScalarFrom`) spawned her too.
- With the queen dead his list was empty, so cleanup skipped his cooldown entries. The static `Dictionary<BaseCreature, long>` on the ability singleton kept a reference to the deleted creature. `MonsterAbilityGroup` children's entries were never cleaned either.
- Neira threw unholy bones from `AlterDamageScalarFrom`, so any caller asking for a damage scalar (a preview gump, for example) could fire the effect.

## Changes
- **Per-creature cooldowns:** a lazy `Dictionary<MonsterAbility, long>` on `BaseCreature` (`IsAbilityOnCooldown`, `SetAbilityCooldown`, `ClearAbilityCooldowns`). The static dictionaries and the query-driven cleanup are gone, and death still resets cooldowns. Tick comparisons are wraparound-safe.
- **Pure contract:** `GetMonsterAbilities()` is documented as pure. The new `virtual bool CanUseAbility(MonsterAbility)` gates abilities that only work some of the time. Every `TriggerAbility*` loop and `GetAbility` check it.
- **LordOaks:** always lists his abilities; `CanUseAbility` and the 10% spell shield read "queen alive". The queen spawns when he is first aggressed or first given a combatant, never on the internal map. Aggression lands before the damage it causes, so the first spell is still shielded, as before.
- **Neira:** the unholy bone throw moves to `OnDamagedBySpell`.

A follow-up PR makes minions such as the queen, pixies and Barracoon's rats belong to their champion spawn, so a spawn reset cleans them up.

## Testing
- `LordOaksTests` covers three cases:
  - querying or deleting an unengaged Lord Oaks spawns nothing;
  - aggression spawns the queen exactly once, and the shield and abilities follow her life;
  - an Oaks on the internal map never spawns her.
- UOContent.Tests: 1234 passed, 2 skipped.
- A local constructor sweep (every Mobile built and deleted repeatedly) no longer reports LordOaks.

Projects/UOContent.Tests/Tests/Mobiles/LordOaksTests.cs

@@ -0,0 +1,98 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using Server;
+using Server.Mobiles;
+using Xunit;
+
+namespace UOContent.Tests.Mobiles;
+
+// Lord Oaks' abilities and spell shield depend on his queen, so the queen must spawn from combat,
+// never from a query such as GetMonsterAbilities or a damage calculation.
+[Collection("Sequential UOContent Tests")]
+public class LordOaksTests : IDisposable
+{
+    private readonly List<Mobile> _created = [];
+
+    public void Dispose()
+    {
+        foreach (var m in World.Mobiles.Values.OfType<Silvani>().ToList())
+        {
+            m.Delete();
+        }
+
+        for (var i = 0; i < _created.Count; i++)
+        {
+            _created[i].Delete();
+        }
+    }
+
+    private LordOaks NewOaks(Map map)
+    {
+        var oaks = new LordOaks();
+        _created.Add(oaks);
+        oaks.MoveToWorld(new Point3D(1000, 1000, 0), map);
+        return oaks;
+    }
+
+    private Mobile NewAttacker()
+    {
+        var attacker = new Mongbat();
+        _created.Add(attacker);
+        return attacker;
+    }
+
+    private static int SilvaniCount() => World.Mobiles.Values.Count(m => m is Silvani);
+
+    [Fact]
+    public void QueryingOrDeletingAnUnengagedOaks_SpawnsNoQueen()
+    {
+        var oaks = NewOaks(Map.Felucca);
+
+        var scalar = 1.0;
+        oaks.AlterDamageScalarFrom(null, ref scalar);
+        oaks.GetMonsterAbilities();
+        oaks.Delete();
+
+        Assert.Equal(1.0, scalar);
+        Assert.Equal(0, SilvaniCount());
+    }
+
+    [Fact]
+    public void Aggression_SpawnsTheQueenOnce_AndShieldsOaksWhileSheLives()
+    {
+        var oaks = NewOaks(Map.Felucca);
+        var attacker = NewAttacker();
+
+        oaks.AggressiveAction(attacker, false);
+        oaks.AggressiveAction(attacker, false);
+
+        var queen = World.Mobiles.Values.OfType<Silvani>().Single();
+
+        var shielded = 1.0;
+        oaks.AlterDamageScalarFrom(attacker, ref shielded);
+        Assert.Equal(0.1, shielded, 6);
+        Assert.True(oaks.CanUseAbility(MonsterAbilities.SummonPixiesCounter));
+
+        queen.Delete();
+
+        var exposed = 1.0;
+        oaks.AlterDamageScalarFrom(attacker, ref exposed);
+        oaks.AggressiveAction(attacker, false);
+
+        Assert.Equal(1.0, exposed);
+        Assert.False(oaks.CanUseAbility(MonsterAbilities.SummonPixiesCounter));
+        Assert.Equal(0, SilvaniCount());
+    }
+
+    [Fact]
+    public void AnOaksOnTheInternalMap_NeverSpawnsTheQueen()
+    {
+        var oaks = new LordOaks();
+        _created.Add(oaks);
+
+        oaks.AggressiveAction(NewAttacker(), false);
+
+        Assert.Equal(0, SilvaniCount());
+    }
+}

Projects/UOContent/Mobiles/Abilities/MonsterAbility.cs

@@ -1,5 +1,4 @@
 using System;
-using System.Collections.Generic;
 using ModernUO.CodeGeneratedEvents;
 
 namespace Server.Mobiles;
@@ -9,8 +8,6 @@ namespace Server.Mobiles;
 /// </summary>
 public abstract class MonsterAbility
 {
-    private Dictionary<BaseCreature, long> _nextTriggerTicks;
-
     public virtual MonsterAbilityType AbilityType => MonsterAbilityType.Generic;
 
     public virtual MonsterAbilityTrigger AbilityTrigger => MonsterAbilityTrigger.None;
@@ -30,18 +27,9 @@ public abstract class MonsterAbility
             return false;
         }
 
-        if (_nextTriggerTicks?.TryGetValue(source, out var nextTrigger) == true)
+        if (source.IsAbilityOnCooldown(this))
         {
-            if (nextTrigger - Core.TickCount > 0)
+            return false;
-            {
-                return false;
-            }
-
-            _nextTriggerTicks.Remove(source);
-            if (_nextTriggerTicks.Count == 0)
-            {
-                _nextTriggerTicks = null;
-            }
         }
 
         var c = ChanceToTrigger;
@@ -65,10 +53,7 @@ public abstract class MonsterAbility
             ? MinTriggerCooldown
             : Utility.RandomMinMax(MinTriggerCooldown, MaxTriggerCooldown);
 
-        var nextTrigger = Core.TickCount + (long)randomAmount.TotalMilliseconds;
+        source.SetAbilityCooldown(this, randomAmount);
-
-        _nextTriggerTicks ??= new Dictionary<BaseCreature, long>();
-        _nextTriggerTicks[source] = nextTrigger;
     }
 
     public virtual void AlterMeleeDamageFrom(BaseCreature source, Mobile target, ref int damage)
@@ -100,18 +85,5 @@ public abstract class MonsterAbility
     }
 
     [OnEvent(nameof(CreatureEvents.CreatureDeathEvent))]
-    [OnEvent(nameof(CreatureEvents.CreatureDeletedEvent))]
+    public static void ResetCooldownsOnDeath(BaseCreature source) => source.ClearAbilityCooldowns();
-    public static void InvalidateNextAbilityTriggers(BaseCreature source)
-    {
-        var abilities = source.GetMonsterAbilities();
-        if (abilities == null || abilities.Length == 0)
-        {
-            return;
-        }
-
-        for (var i = 0; i < abilities.Length; i++)
-        {
-            abilities[i]._nextTriggerTicks?.Remove(source);
-        }
-    }
 }

Projects/UOContent/Mobiles/BaseCreature.cs

@@ -1,4 +1,4 @@
-using System;
+using System;
 using System.Collections.Generic;
 using System.Runtime.CompilerServices;
 using ModernUO.Serialization;
@@ -1500,10 +1500,47 @@ namespace Server.Mobiles
         public List<MLQuest> MLQuests =>
             (m_MLQuests ??= StaticMLQuester ? MLQuestSystem.FindQuestList(GetType()) : ConstructQuestList()) ?? MLQuestSystem.EmptyList;
 
+        /// <summary>
+        ///     The abilities this creature has. Must be a pure, stable query: it is called from lookups, damage
+        ///     calculations and cleanup, so it must not spawn, mutate state, or change with the creature's state.
+        ///     Gate abilities that should only work some of the time with <see cref="CanUseAbility" />.
+        /// </summary>
         public virtual MonsterAbility[] GetMonsterAbilities() => null;
 
+        /// <summary>
+        ///     Whether <paramref name="ability" /> may trigger or be looked up right now. Must be a pure read.
+        /// </summary>
+        public virtual bool CanUseAbility(MonsterAbility ability) => true;
+
         private MonsterAbilityTrigger _activeTriggers;
 
+        // Deadlines in Core.TickCount, keyed by ability. Lives on the creature so it is collected with it.
+        private Dictionary<MonsterAbility, long> _abilityCooldowns;
+
+        public bool IsAbilityOnCooldown(MonsterAbility ability)
+        {
+            if (_abilityCooldowns?.TryGetValue(ability, out var nextTrigger) != true)
+            {
+                return false;
+            }
+
+            if (nextTrigger - Core.TickCount > 0)
+            {
+                return true;
+            }
+
+            _abilityCooldowns.Remove(ability);
+            return false;
+        }
+
+        public void SetAbilityCooldown(MonsterAbility ability, TimeSpan cooldown)
+        {
+            _abilityCooldowns ??= new Dictionary<MonsterAbility, long>();
+            _abilityCooldowns[ability] = Core.TickCount + (long)cooldown.TotalMilliseconds;
+        }
+
+        public void ClearAbilityCooldowns() => _abilityCooldowns = null;
+
         public virtual MonsterAbility GetAbility(MonsterAbilityType type)
         {
             var abilities = GetMonsterAbilities();
@@ -1516,6 +1553,11 @@ namespace Server.Mobiles
             for (var i = 0; i < abilities.Length; i++)
             {
                 var ability = abilities[i];
+                if (!CanUseAbility(ability))
+                {
+                    continue;
+                }
+
                 if (ability is MonsterAbilityGroup group)
                 {
                     ability = group.GetAbilityWithType(type);
@@ -1579,7 +1621,7 @@ namespace Server.Mobiles
                 for (var i = 0; i < abilities.Length; i++)
                 {
                     var ability = abilities[i];
-                    if (ability.CanTrigger(this, trigger))
+                    if (CanUseAbility(ability) && ability.CanTrigger(this, trigger))
                     {
                         ability.Trigger(trigger, this, defender);
                         triggered = true;
@@ -1606,7 +1648,7 @@ namespace Server.Mobiles
             for (var i = 0; i < abilities.Length; i++)
             {
                 var ability = abilities[i];
-                if (ability.CanTrigger(this, trigger))
+                if (CanUseAbility(ability) && ability.CanTrigger(this, trigger))
                 {
                     ability.Move(this, d);
                 }
@@ -1625,7 +1667,7 @@ namespace Server.Mobiles
             for (var i = 0; i < abilities.Length; i++)
             {
                 var ability = abilities[i];
-                if (ability.CanTrigger(this, trigger))
+                if (CanUseAbility(ability) && ability.CanTrigger(this, trigger))
                 {
                     if ((trigger & MonsterAbilityTrigger.GiveMeleeDamage) != 0)
                     {
@@ -1666,7 +1708,7 @@ namespace Server.Mobiles
             for (var i = 0; i < abilities.Length; i++)
             {
                 var ability = abilities[i];
-                if (ability.CanTrigger(this, trigger))
+                if (CanUseAbility(ability) && ability.CanTrigger(this, trigger))
                 {
                     if ((trigger & MonsterAbilityTrigger.GiveSpellDamage) != 0)
                     {

Projects/UOContent/Mobiles/Special/LordOaks.cs

@@ -86,7 +86,11 @@ public partial class LordOaks : BaseChampion
     public override Poison PoisonImmune => Poison.Deadly;
 
     private static MonsterAbility[] _abilities = { MonsterAbilities.SummonPixiesCounter };
-    public override MonsterAbility[] GetMonsterAbilities() => CheckQueen() ? _abilities : MonsterAbilities.Empty;
+    public override MonsterAbility[] GetMonsterAbilities() => _abilities;
+
+    public override bool CanUseAbility(MonsterAbility ability) => IsQueenAlive;
+
+    private bool IsQueenAlive => _queen?.Deleted == false;
 
     public override void GenerateLoot()
     {
@@ -103,36 +107,41 @@ public partial class LordOaks : BaseChampion
 
     public override int GetDeathSound() => 0x2F7;
 
-    public bool CheckQueen()
+    // Aggression lands before the damage it causes, so the queen is up before the first hit is scaled.
+    public override void AggressiveAction(Mobile aggressor, bool criminal)
     {
-        if (Map == null)
+        base.AggressiveAction(aggressor, criminal);
+        SpawnQueen();
+    }
+
+    public override void OnCombatantChange()
+    {
+        base.OnCombatantChange();
+
+        if (Combatant != null)
         {
-            return false;
+            SpawnQueen();
+        }
+    }
+
+    private void SpawnQueen()
+    {
+        if (_spawnedQueen || Deleted || Map == null || Map == Map.Internal)
+        {
+            return;
         }
 
-        if (!_spawnedQueen)
+        Say(1042153); // Come forth my queen!
-        {
-            Say(1042153); // Come forth my queen!
 
-            _queen = new Silvani { Team = Team };
+        Queen = new Silvani { Team = Team };
-            _queen.MoveToWorld(Location, Map);
+        Queen.MoveToWorld(Location, Map);
 
-            _spawnedQueen = true;
+        SpawnedQueen = true;
-            return true;
-        }
-
-        if (_queen?.Deleted != false)
-        {
-            _queen = null;
-            return false;
-        }
-
-        return true;
     }
 
     public override void AlterDamageScalarFrom(Mobile caster, ref double scalar)
     {
-        if (CheckQueen())
+        if (IsQueenAlive)
         {
             scalar *= 0.1;
         }

Projects/UOContent/Mobiles/Special/Neira.cs

@@ -163,13 +163,13 @@ public partial class Neira : BaseChampion
         }
     }
 
-    public override void AlterDamageScalarFrom(Mobile caster, ref double scalar)
+    public override void OnDamagedBySpell(Mobile from, int damage)
     {
-        base.AlterDamageScalarFrom(caster, ref scalar);
+        base.OnDamagedBySpell(from, damage);
 
         if (Utility.RandomDouble() < 0.1) // 10% chance to throw an unholy bone
         {
-            AddUnholyBone(caster, 1.0);
+            AddUnholyBone(from, 1.0);
         }
     }