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).

dev-docs/runuo-migration-docs/09-items-mobiles-creatures.md

@@ -497,6 +497,49 @@ An isolated step (after the creature stood for at least a walk interval) goes ou
 walk regardless of pace — only a continuing cadence, or a pace faster than the run
 interpolation, flags run.
 
+## Retaliation: `OnAggressiveAction` Returns `bool`
+
+RunUO's `BaseAI.OnAggressiveAction` was a `void` hook that only swapped to a closer
+aggressor; `BaseCreature.AggressiveAction` then decided the response itself and set
+`Combatant` regardless of what the AI had done. ModernUO moves the whole retaliation policy
+into the hook — the stand-down verdict for commanded pets, the closer-aggressor swap
+(`PreferCloserAggressor`), and the `IssueOrder(Attack)` vs `Combatant =` choice — and
+`BaseCreature` only runs its bookkeeping (`StopFlee`, `ForceReacquire`, ethics shield) when
+the hook returns `true`:
+
+```csharp
+// RunUO — swap only; BaseCreature picks the combatant afterwards
+public override void OnAggressiveAction(Mobile aggressor)
+{
+    if (m_Mobile.Combatant == null)
+    {
+        m_Mobile.Combatant = aggressor;
+        return;
+    }
+
+    base.OnAggressiveAction(aggressor);
+}
+
+// ModernUO — the AI owns the response; false = stood down, nothing else fires
+public override bool OnAggressiveAction(Mobile aggressor)
+{
+    if (Mobile.Combatant == null)
+    {
+        Mobile.Combatant = aggressor;
+    }
+    else
+    {
+        PreferCloserAggressor(aggressor);
+    }
+
+    return true;
+}
+```
+
+An override that used to `return;` early to refuse a fight now returns `false`; calling
+`base.OnAggressiveAction` runs the full stock policy, including setting `Combatant`, so an AI
+that only wants the closer-aggressor swap calls `PreferCloserAggressor` instead.
+
 ## Target Acquisition: `AcquireOnApproach` Is a Delay
 
 RunUO's `AcquireOnApproach` bool (paragon insta-aggro on approach) is now