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.Shared/Polymorph/PolymorphPrototype.cs

@@ -97,6 +97,14 @@ namespace Content.Shared.Polymorph
 
         [DataField("allowRepeatedMorphs", serverOnly: true)]
         public bool AllowRepeatedMorphs = false;
+
+        /// <summary>
+        /// The amount of time that should pass after this polymorph has ended, before a new one
+        /// can occur.
+        /// </summary>
+        [DataField("cooldown", serverOnly: true)]
+        [ViewVariables(VVAccess.ReadWrite)]
+        public TimeSpan Cooldown = TimeSpan.Zero;
     }
 
     public enum PolymorphInventoryChange : byte