fix(GRO-985): fix Messages test mocks and scrollIntoView guard

- Wrap conversation mocks in { items, nextCursor } response shape
  (loadConversations reads json.items, bare array caused undefined.length crash)
- Guard scrollIntoView with ?. (jsdom doesn't implement it)
- Use getAllByText for text appearing in both preview and thread

Co-Authored-By: Paperclip <noreply@paperclip.ing>

apps/web/src/pages/Messages.tsx

@@ -93,7 +93,7 @@ export function MessagesPage() {
 
   useEffect(() => {
     if (messages.length > 0) {
-      messagesEndRef.current?.scrollIntoView({ behavior: "smooth" });
+      messagesEndRef.current?.scrollIntoView?.({ behavior: "smooth" });
     }
   }, [messages]);