Fix robusted dionas not being recoverable (#21636)

Fix robusted dionas not being recoverable (#21636)

* Fix robusted dionas not being recoverable

* Implement a polymorph cooldown field, used for TreeMorph

Allows the polymorphed Diona some time to work off robust harvest
without immediately polymorphing again.

* Minor adjustment to polymorph cooldown timing

* Apply suggestions from code review

Co-authored-by: Kara <lunarautomaton6@gmail.com>
* fix: trigger polymorph revert on destruction

As suggested by mirrorcult, DamageThresholdReached is not the ideal
candidate for reverting a polymorph on destruction, as there exists a
DestructionEventArgs to listen for instead.

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Co-authored-by: Kara <lunarautomaton6@gmail.com>