fix(ai): guard zero-int detection cooldown and duplicate pet friends (#2667)

Supersedes #2658 (rebased onto main; edits by maintainers were not enabled on the original branch). Credit to @MithrilHammer for both fixes and the regression tests; their three commits are kept as authored.

Addresses two items from #2627:

- The detect-hidden cooldown clamps its divisor to at least one. A creature whose effective intelligence reaches zero now uses the existing capped cooldown instead of throwing.
- Pet friends are unique. `AddPetFriend` ignores an existing friend, so one removal revokes the membership.

On top of that, `AddPetFriend` and `RemovePetFriend` now go through the generated `AddToFriends` / `RemoveFromFriends`, which mark the creature dirty. `Friends` goes back to null when the last friend is removed, and removing a non-friend no longer touches the list.

Duplicates already stored in saves are not migrated.

## Tests

`DetectHiddenTimerTests` (7 cases) drives the real AI timer and checks cooldown bounds and retry timing. `PetFriendMembershipTests` (3 cases) covers duplicate adds, removal, and re-adding without disturbing other friends. `UOContent.Tests`: 1158 passed, 2 skipped, 0 failed on Windows.

Projects/UOContent/Mobiles/AI/BaseAI/AITimer.cs

@@ -225,7 +225,7 @@ public sealed class AITimer : Timer
 
     private void CacheDetectHiddenDelays()
     {
-        var delay = Math.Min(30000 / _owner.Mobile.Int, 120);
+        var delay = Math.Min(30000 / Math.Max(1, _owner.Mobile.Int), 120);
         _detectHiddenMinDelay = delay * 900;  // 26s to 108s
         _detectHiddenMaxDelay = delay * 1100; // 32s to 132s
     }

Projects/UOContent/Mobiles/BaseCreature.cs

@@ -4304,16 +4304,27 @@ namespace Server.Mobiles
 
         public virtual void AddPetFriend(Mobile m)
         {
+            if (IsPetFriend(m))
+            {
+                return;
+            }
+
             Friends ??= [];
-            Friends.Add(m);
-            this.MarkDirty();
+            AddToFriends(m);
         }
 
         public virtual void RemovePetFriend(Mobile m)
         {
-            if (Friends?.Remove(m) == true)
+            if (Friends == null)
             {
-                this.MarkDirty();
+                return;
+            }
+
+            RemoveFromFriends(m);
+
+            if (Friends.Count == 0)
+            {
+                Friends = null;
             }
         }