fix(ai): meer mages and their enraged creatures no longer fight each other (#2673)

## Summary

A meer mage fights `FightMode.Evil` and its enraged creatures carry -1000 karma, so `BaseCreature.IsEnemy` made each an enemy of the other:
- The meer acquired its own enraged creatures as ordinary targets.
- An enraged creature fell back to attacking its meer when nothing else was in range. It isn't `Summoned`, so it reached the meer through `AcquireNewFocusMob`'s summon-master fallback.

`BaseEnraged` and `MeerMage` now override `IsEnemy` to exclude each other. Another meer's creatures stay enemies.

`BaseEnraged` also pulled its meer into fights against any creature with a `SummonMaster`, which included another meer's enraged creatures. It now checks `GetMaster()`, matching the "controlled/summoned" OSI note in that method.

## Test plan

- [x] `MeerAndItsEnragedCreature_AreNotEnemies`: fails before the fix
- [x] `AnotherMeersEnragedCreature_StaysAnEnemy`
- [x] `MeerAndItsEnragedCreature_NeverAcquireEachOther`: fails before the fix (the meer acquired its own creature)
- [x] UOContent.Tests (1171 passed, 2 skipped) and Server.Tests (907) green

Projects/UOContent.Tests/Tests/Mobiles/AI/MeerEnragedTests.cs

@@ -0,0 +1,77 @@
+using System;
+using System.Collections.Generic;
+using Server;
+using Server.Mobiles;
+using Xunit;
+
+namespace UOContent.Tests.Mobiles.AI;
+
+// A meer mage fights FightMode.Evil and its enraged creatures carry -1000 karma, so the karma rules alone
+// make each an enemy of the other.
+[Collection("Sequential UOContent Tests")]
+public class MeerEnragedTests : IDisposable
+{
+    private readonly List<Mobile> _created = [];
+
+    public void Dispose()
+    {
+        for (var i = 0; i < _created.Count; i++)
+        {
+            _created[i].Delete();
+        }
+    }
+
+    private T Track<T>(T m) where T : Mobile
+    {
+        _created.Add(m);
+        return m;
+    }
+
+    private static void Place(BaseCreature bc, int x)
+    {
+        var map = Map.Felucca;
+        bc.MoveToWorld(new Point3D(x, 1600, map.GetAverageZ(x, 1600)), map); // flat, clear line of sight both ways
+        bc.AIObject.AITimer.Stop();
+    }
+
+    private static bool Acquire(BaseCreature creature)
+    {
+        creature.NextReacquireTime = Core.TickCount - 1;
+        return creature.AIObject.AcquireFocusMob(creature.RangePerception, creature.FightMode, false, false, true);
+    }
+
+    [Fact]
+    public void MeerAndItsEnragedCreature_AreNotEnemies()
+    {
+        var meer = Track(new MeerMage());
+        var enraged = Track(new EnragedRabbit(meer));
+
+        Assert.False(enraged.IsEnemy(meer));
+        Assert.False(meer.IsEnemy(enraged));
+    }
+
+    [Fact]
+    public void AnotherMeersEnragedCreature_StaysAnEnemy()
+    {
+        var meer = Track(new MeerMage());
+        var other = Track(new MeerMage());
+        var enraged = Track(new EnragedRabbit(other));
+
+        Assert.True(enraged.IsEnemy(meer));
+        Assert.True(meer.IsEnemy(enraged));
+    }
+
+    [SkippableFact]
+    public void MeerAndItsEnragedCreature_NeverAcquireEachOther()
+    {
+        Skip.If(!Server.Tests.TestServerInitializer.TileDataLoaded, "Requires UO client map data.");
+
+        var meer = Track(new MeerMage());
+        var enraged = Track(new EnragedBlackBear(meer));
+        Place(meer, 1600);
+        Place(enraged, 1601);
+
+        Assert.False(Acquire(enraged));
+        Assert.False(Acquire(meer));
+    }
+}

Projects/UOContent/Mobiles/Monsters/LBR/Meers/EnragedCreatures.cs

@@ -95,6 +95,9 @@ namespace Server.Mobiles
             SummonMaster = summoner;
         }
 
+        // The meer's FightMode.Evil against our negative karma would otherwise make us enemies.
+        public override bool IsEnemy(Mobile m) => m != SummonMaster && base.IsEnemy(m);
+
         public override void OnThink()
         {
             if (SummonMaster?.Deleted != false)
@@ -118,7 +121,7 @@ namespace Server.Mobiles
             */
             else if (!Combat(SummonMaster))
             {
-                if (Combatant.Player || Combatant is BaseCreature bc && (bc.Controlled || bc.SummonMaster != null))
+                if (Combatant.Player || Combatant is BaseCreature bc && bc.GetMaster() != null)
                 {
                     SummonMaster.Combatant = Combatant;
                 }

Projects/UOContent/Mobiles/Monsters/LBR/Meers/MeerMage.cs

@@ -59,6 +59,9 @@ namespace Server.Mobiles
 
         public override bool InitialInnocent => true;
 
+        // FightMode.Evil would otherwise target our own enraged creatures for their negative karma.
+        public override bool IsEnemy(Mobile m) => (m as BaseEnraged)?.SummonMaster != this && base.IsEnemy(m);
+
         public override void GenerateLoot()
         {
             AddLoot(LootPack.FilthyRich);