fix(chat): improve error message for missing message

- Updated the error message in `MessageService` to provide a clearer indication when a message with a specific `leaf_id` is not found, enhancing the clarity of error reporting.

Author: danyalxahid-askui

src/askui/chat/api/messages/service.py

@@ -167,7 +167,7 @@ def _get_path_endpoints(
             ).scalar_one_or_none()
 
             if branch_root_id is None:
-                error_msg = f"Root message not found in thread {thread_id} for message {leaf_id}"
+                error_msg = f"Message with id '{leaf_id}' not found"
                 raise NotFoundError(error_msg)
 
         else: