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.

---------

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

Content.Server/Polymorph/Components/PolymorphableComponent.cs

@@ -12,6 +12,12 @@ namespace Content.Server.Polymorph.Components
         /// </summary>
         public Dictionary<string, EntityUid>? PolymorphActions = null;
 
+        /// <summary>
+        /// Timestamp for when the most recent polymorph ended.
+        /// </summary>
+        [ViewVariables(VVAccess.ReadOnly)]
+        public TimeSpan? LastPolymorphEnd = null;
+
         /// <summary>
         /// The polymorphs that the entity starts out being able to do.
         /// </summary>