Support for long-running background after original `EventQueue`

### Is your feature request related to a problem? Please describe.

**Description:**
When using the `EventQueue` in the current A2A examples, the queue is closed automatically after the request completes. This causes issues for long-running background tasks (like reminders or notifications) that attempt to enqueue events after the request has finished.

**Observed behavior:**

* Background tasks finish successfully.
* Attempting to enqueue events via the original `EventQueue` results in warnings like:

```
Queue is closed. Event will not be enqueued.
```
* Any updates or artifacts pushed after the request cannot be delivered through the closed queue.

**Context / use case:**
This is needed for features like reminders, scheduled notifications, or other tasks that run asynchronously and complete after the original agent request has finished.

---

**Questions:**

* Does A2A currently support this pattern?
* Can this behavior be reproduced using the protocol as it stands?
* Are there any recommended examples or best practices for implementing long-running background tasks that deliver updates back to the client?

Thank you!

### Describe the solution you'd like

* Support a mechanism for long-running tasks to send updates or final artifacts back to the client even after the request has completed.

### Describe alternatives you've considered

_No response_

### Additional context

_No response_

### Code of Conduct

- [x] I agree to follow this project's Code of Conduct