DEV: When deleting a chat message, do not delete mention records (#21486)

A chat message may be restored later, so we shouldn't be deleting `chat_mentions` records for it.

But we still have to remove notifications (see 082cd139).

plugins/chat/app/services/chat/trash_message.rb

@@ -22,7 +22,7 @@ module Chat
     policy :invalid_access
     transaction do
       step :trash_message
-      step :destroy_mentions
+      step :destroy_notifications
       step :update_tracking_state
       step :update_thread_reply_cache
     end
@@ -53,8 +53,10 @@ module Chat
       message.trash!
     end
 
-    def destroy_mentions(message:, **)
-      Chat::Mention.where(chat_message: message).destroy_all
+    def destroy_notifications(message:, **)
+      ids = Chat::Mention.where(chat_message: message).pluck(:notification_id)
+      Notification.where(id: ids).destroy_all
+      Chat::Mention.where(chat_message: message).update_all(notification_id: nil)
     end
 
     def update_tracking_state(message:, **)

plugins/chat/spec/services/chat/trash_message_spec.rb

@@ -43,10 +43,15 @@ RSpec.describe Chat::TrashMessage do
           expect(Chat::Message.find_by(id: message.id)).to be_nil
         end
 
-        it "destroys associated mentions" do
-          mention = Fabricate(:chat_mention, chat_message: message)
+        it "destroys notifications for mentions" do
+          notification = Fabricate(:notification)
+          mention = Fabricate(:chat_mention, chat_message: message, notification: notification)
+
           result
-          expect(Chat::Mention.find_by(id: mention.id)).to be_nil
+
+          mention = Chat::Mention.find_by(id: mention.id)
+          expect(mention).to be_present
+          expect(mention.notification_id).to be_nil
         end
 
         it "publishes associated Discourse and MessageBus events" do