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/AI/BaseAI/BaseAI.cs

@@ -137,6 +137,12 @@ public abstract partial class BaseAI
     /// </summary>
     public virtual bool OnAggressiveAction(Mobile aggressor)
     {
+        if (Mobile.RefusesGuardTarget(aggressor))
+        {
+            DebugSay("Friendly fire; I keep guarding.");
+            return false;
+        }
+
         // Only a creature somebody can command has been told anything; wild creatures rest on None.
         var toldToStandDown = Mobile.Controlled && Mobile.ControlMaster != null && Mobile.Commandable &&
                               IsStandDownOrder(Mobile.ControlOrder);
@@ -856,7 +862,19 @@ public abstract partial class BaseAI
             return false;
         }
 
-        if (HandleBardProvoked() || HandleControlled() || HandleConstantFocus())
+        if (HandleBardProvoked())
+        {
+            return true;
+        }
+
+        // A controlled creature must not fall through to wild target selection when
+        // its order has no target. Guard in particular normally has no ControlTarget.
+        if (Mobile.Controlled)
+        {
+            return HandleControlled();
+        }
+
+        if (HandleConstantFocus())
         {
             return true;
         }
@@ -911,8 +929,17 @@ public abstract partial class BaseAI
 
     private bool HandleControlled()
     {
-        if (!Mobile.Controlled)
+        if (Mobile.ControlOrder == OrderType.Guard)
         {
+            Mobile.FocusMob = FindGuardTarget();
+            return Mobile.FocusMob != null;
+        }
+
+        // Follow and other non-combat orders may target the owner or a friend.
+        // Only an explicit attack order supplies a combat target.
+        if (Mobile.ControlOrder != OrderType.Attack)
+        {
+            Mobile.FocusMob = null;
             return false;
         }