fixup! Enforce MAX_COMMENT_DEPTH in discussion_api

Add more extensive testing

lms/djangoapps/discussion_api/serializers.py

@@ -303,7 +303,7 @@ class CommentSerializer(_ContentSerializer):
                     "parent_id does not identify a comment in the thread identified by thread_id."
                 )
         if is_comment_too_deep(parent):
-            raise ValidationError({"parent_id": ["Parent is too deep."]})
+            raise ValidationError({"parent_id": ["Comment level is too deep."]})
         return attrs
 
     def restore_object(self, attrs, instance=None):