FIX: Do not serialize thread data if threading disabled (#21107)

Followup to ba11cf4767,
this commit makes it so that none of the chat message
thread data is serialized if threading_enabled is false
for the channel or if enable_experimental_chat_threaded_discussions
is false, there is no need to serialize the data in this
case.

plugins/chat/app/serializers/chat/message_serializer.rb

@@ -153,8 +153,16 @@ module Chat
       end
     end
 
+    def include_threading_data?
+      SiteSetting.enable_experimental_chat_threaded_discussions && channel.threading_enabled
+    end
+
+    def include_thread_id?
+      include_threading_data?
+    end
+
     def include_thread_reply_count?
-      object.thread_id.present?
+      include_threading_data? && object.thread_id.present?
     end
 
     def thread_reply_count