DEV: do not fabricate a Notification when fabricating a ChatMention (#20450)

Initially, the chat_mention db table was created to support notifications. So when creating 
a `chat_mention` record we were always creating a related `notification` record. So did the
ChatMention fabricator. 

Now we want to use the chat_mention db table in other scenarios. So we started decoupling 
mentions from notification in 75b81b68.

This removes fabrication of Notifications from the ChatMention fabricator. We need to be able 
to fabricate a ChatMention without a Notification.

plugins/chat/spec/lib/message_mover_spec.rb

@@ -109,7 +109,14 @@ describe Chat::MessageMover do
     it "updates references for reactions, uploads, revisions, mentions, etc." do
       reaction = Fabricate(:chat_message_reaction, chat_message: message1)
       upload = Fabricate(:upload_reference, target: message1)
-      mention = Fabricate(:chat_mention, chat_message: message2, user: acting_user)
+      notification = Fabricate(:notification)
+      mention =
+        Fabricate(
+          :chat_mention,
+          chat_message: message2,
+          user: acting_user,
+          notification: notification,
+        )
       revision = Fabricate(:chat_message_revision, chat_message: message3)
       webhook_event = Fabricate(:chat_webhook_event, chat_message: message3)
       move!