FIX: Chat publisher publishing to thread when threading disabled (#21179)

Followup to bd5c5c4b5f, a
bug was introduced there for any channel that did not have
threading enabled or sites with the experimental threading
disabled. When the user replied to another chat message,
since this is always a thread in the background, we weren't
sending any MessageBus messages to the main channel, since
the message was a thread reply.

However in the UI these messages still show in the main stream
of the channel if threading is turned off, so the UI was not
reacting to these things happening in the backend. The worst
issue was that new clients would not see new replies sent in
reply to other messages in the channel.

plugins/chat/app/services/chat/publisher.rb

@@ -15,18 +15,22 @@ module Chat
     end
 
     def self.calculate_publish_targets(channel, message)
-      targets =
-        if message.thread_om?
-          [
-            root_message_bus_channel(channel.id),
-            thread_message_bus_channel(channel.id, message.thread_id),
-          ]
-        elsif message.thread_reply?
-          [thread_message_bus_channel(channel.id, message.thread_id)]
-        else
-          [root_message_bus_channel(channel.id)]
-        end
-      targets
+      return [root_message_bus_channel(channel.id)] if !allow_publish_to_thread?(channel)
+
+      if message.thread_om?
+        [
+          root_message_bus_channel(channel.id),
+          thread_message_bus_channel(channel.id, message.thread_id),
+        ]
+      elsif message.thread_reply?
+        [thread_message_bus_channel(channel.id, message.thread_id)]
+      else
+        [root_message_bus_channel(channel.id)]
+      end
+    end
+
+    def self.allow_publish_to_thread?(channel)
+      SiteSetting.enable_experimental_chat_threaded_discussions && channel.threading_enabled
     end
 
     def self.publish_new!(chat_channel, chat_message, staged_id)
@@ -40,7 +44,7 @@ module Chat
       # NOTE: This means that the read count is only updated in the client
       # for new messages in the main channel stream, maybe in future we want to
       # do this for thread messages as well?
-      if !chat_message.thread_reply?
+      if !chat_message.thread_reply? || !allow_publish_to_thread?(chat_channel)
         MessageBus.publish(
           self.new_messages_message_bus_channel(chat_channel.id),
           {