FIX: streaming related specs (#1448)

## 🔍 Overview
This update fixes an issue where message bus streaming related specs
were not working correctly. To do so we pass the `last_id` when
subscribing to `MessageBus` which allows us to unskip those broken
tests.

---------

Co-authored-by: Joffrey JAFFEUX <j.jaffeux@gmail.com>

assets/javascripts/discourse/components/modal/diff-modal.gjs

@@ -85,14 +85,18 @@ export default class ModalDiffModal extends Component {
 
   @bind
   subscribe() {
-    this.messageBus.subscribe(CHANNEL, this.updateResult);
+    this.messageBus.subscribe(
+      CHANNEL,
+      this.updateResult,
+      this.currentUser
+        ?.discourse_ai_helper_stream_composer_suggestion_last_message_bus_id
+    );
   }
 
   @bind
   cleanup() {
     // stop all callbacks so it does not end up streaming pointlessly
-    this.smoothStreamer.resetStreaming();
-    this.diffStreamer.reset();
+    this.#resetState();
     this.messageBus.unsubscribe(CHANNEL, this.updateResult);
   }