fix(ai): guarding pets never target their owner or the owner's pets (#2665)

Supersedes #2657 (rebased onto #2662, which the original branch conflicted with). Credit to @MithrilHammer for the diagnosis, the fix and the regression tests; their three commits are kept as authored.

Fixes #2656.

## Root cause

When a guarding pet's combatant hides or becomes invalid, its AI calls `AcquireFocusMob` to find a new target. A controlled creature with no `ControlTarget` (Guard normally has none) fell out of `HandleControlled` and into the wild-creature scan, where `IsEnemy` treats any player as an enemy. With `FightMode.Closest`, the owner won. The fall-through dates to #2232. RunUO never let a controlled creature reach the wild scan.

## Changes

- `AcquireFocusMob`: a controlled creature returns whatever `HandleControlled` decides. Guard uses `FindGuardTarget`, Attack uses its `ControlTarget`, and every other order acquires nothing.
- `FindGuardTarget` skips guard allies: the master and creatures the master controls, including controlled summons. Uncontrolled summons (energy vortex, blade spirits) attack their caster, so a guard still defends against them, as in RunUO.
- Friendly fire: `BaseAI.OnAggressiveAction` stands a guard down when an ally is the aggressor. `BaseCreature.Combatant` refuses an ally while guarding, because `Mobile.AggressiveAction` assigns the first aggressor as `Combatant` before any retaliation policy runs. The veto sits on `Combatant` rather than `CanBeHarmful`, so what a pet can harm doesn't depend on its current order. An explicit Attack order and bard provocation can still target an ally.

## Tests

16 regression cases in `GuardFriendlyFireTests` (skipped without client map data), including a real `WhiteWyrm` reproduction. `UOContent.Tests`: 1142 passed, 2 skipped, 0 failed on Windows.

Projects/UOContent/Mobiles/BaseCreature.cs

@@ -3650,6 +3650,26 @@ namespace Server.Mobiles
             m?.InvalidateProperties();
         }
 
+        // A guard never fights its master or the master's other pets. An explicit Attack order and bard
+        // provocation still may.
+        public bool RefusesGuardTarget(Mobile target) =>
+            _controlled && _controlOrder == OrderType.Guard && !BardProvoked && AIObject?.IsGuardAlly(target) == true;
+
+        // Mobile.AggressiveAction assigns the first aggressor as Combatant before any retaliation policy runs.
+        public override Mobile Combatant
+        {
+            get => base.Combatant;
+            set
+            {
+                if (value != null && RefusesGuardTarget(value))
+                {
+                    return;
+                }
+
+                base.Combatant = value;
+            }
+        }
+
         public override bool CanBeHarmful(Mobile target, bool message, bool ignoreOurBlessedness)
         {
             if (target is BaseFactionGuard)