fix: event doesn't have messageId

Author: acensia

src/a2a/server/tasks/task_manager.py

@@ -134,10 +134,10 @@ async def save_task_event(
             if task.status.message:
                 if not task.history:
                     task.history = [task.status.message]
-                elif task.history[-1].messageId != event.status.message.messageId:
+                elif task.history[-1].messageId != task.status.message.messageId:
                     task.history.append(task.status.message)
                 elif task.status.message.isDelta:
-                    task.history[-1].parts.extend(event.status.message.parts)
+                    task.history[-1].parts.extend(task.status.message.parts)
             if event.metadata:
                 if not task.metadata:
                     task.metadata = {}