fix(ai): BaseAI.OnAggressiveAction owns the retaliation policy; familiars refuse cleanly (#2662)

> [!IMPORTANT]
> **Breaking change to API to fix NPC AI issues**

## Symptom

`FamiliarAITests.HiddenCaster_FamiliarRefusesRetaliation` fails on main at `Assert.False(wolf.Warmode)`: the familiar's `Combatant` stays null as intended, but it is left in war stance. In-game the familiar of a hidden caster flips into warmode for a tick when struck, until `FamiliarAI.StandDown()` clears it on the next think.

CI never saw it because the familiar tests are `SkippableFact` (they need client tiledata).

## Root cause

#2644 and #2645 landed a day apart and are individually correct but conflict at one seam.

Since #2645, a familiar (`Commandable == false`) is no longer "told to stand down", so `BaseCreature.AggressiveAction` continues past `AIObject.OnAggressiveAction(aggressor)` to its generic fallback:

```cs
else if (Combatant == null && !BardPacified)
{
    Warmode = true;
    Combatant = aggressor;
}
```

`FamiliarAI.OnAggressiveAction` had already declined (caster hidden), but the hook was `void`, so the creature could not tell. The fallback raised `Warmode` before `BaseFamiliar.Combatant` vetoed the target.

## Fix

This is the follow-up agreed in #2645: the retaliation policy moves into a virtual `BaseAI.OnAggressiveAction(aggressor) -> bool`.

- **`BaseAI.OnAggressiveAction`** now holds the stand-down verdict, the closer-aggressor swap (`PreferCloserAggressor`), and the `IssueOrder(Attack)` vs `Combatant =` decision. Returns `false` when the creature stands down.
- **`BaseCreature.AggressiveAction`** keeps only bookkeeping — `ControlMaster.Aggressors` entry, `StopFlee`, `ForceReacquire`, ethics `FinishShield` — keyed off the return, so an AI that declines is never second-guessed.
- **`FamiliarAI`** overrides the whole policy: no orders to stand down on, never fights for a hidden caster or a hidden aggressor.

Behaviour for every other AI is unchanged. Moving the fallback ahead of `StopFlee`/`ForceReacquire` is order-safe: both are pure field resets, `IssueAttack` does not `Think()`, and `OnCombatantChange` only derives `Warmode`.

Two commits: the first is the minimal `bool` hook that alone fixes the test, the second the policy move.

## Not changed (noted for later)

- `Warmode = true; Combatant = x;` (also in `FamiliarAI.Act`, `IssueAttack`) is redundant when the setter accepts — `BaseCreature.OnCombatantChange` derives `Warmode` — and is the dangling-warmode hazard whenever a `Combatant` setter vetoes.
- Since #2614 a stood-down pet skips the ethics `FinishShield`; RunUO ran it unconditionally. Preserved as-is.

## Tests

`HiddenCaster_FamiliarRefusesRetaliation` fails on main and passes here. Full `UOContent.Tests`: 1126 passed, 0 failed, 2 skipped (spawner fixture capture + benchmark).

Projects/UOContent/Mobiles/AI/FamiliarAI.cs

@@ -47,21 +47,25 @@ public class FamiliarAI : BaseAI
         return Mobile.Controlled ? OrderType.Come : OrderType.None;
     }
 
-    public override void OnAggressiveAction(Mobile aggressor)
+    // No orders to stand down on; never fights for a hidden caster.
+    public override bool OnAggressiveAction(Mobile aggressor)
     {
         if (!Familiar.AssistsMaster || aggressor.Hidden || Familiar.ControlMaster?.Hidden == true)
         {
-            return;
+            return false;
         }
 
         if (Mobile.Combatant == null)
         {
             Mobile.Warmode = true;
             Mobile.Combatant = aggressor;
-            return;
+        }
+        else
+        {
+            PreferCloserAggressor(aggressor);
         }
 
-        base.OnAggressiveAction(aggressor);
+        return true;
     }
 
     private bool Act()