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.
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