Validate new blocks during copy

It is possible that a block is copied prior to validation, in which case
some fields (like when) which should be something other than a string might
not be. Using validate() in copy() is relatively harmless and ensures the
blocks are in the proper structure.

This also cleans up some of the finalized logic from an earlier commit and
adds similar logic for validated.

Fixes #17018

lib/ansible/playbook/block.py

@@ -192,6 +192,7 @@ class Block(Base, Become, Conditional, Taggable):
         if self._role:
             new_me._role = self._role
 
+        new_me.validate()
         return new_me
 
     def serialize(self):