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

Look at the A2A samples repository for examples using EventQueue, particularly how queues are opened and closed. Examine the protocol specification for lifecycle management. The goal is to understand the current closure behavior and design a mechanism for persistent or reconnectable queues for background tasks.

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