Ensure that task results are delivered during pool shutdown

[weetster]

This fix removes a call to result_handler.terminate() to ensure that the ResultHandler thread stays running and processes any task results in its queue before shutting down. The thread will terminate itself naturally after its queue is emptied.

I've added a unit test to exercise this behaviour during pool shutdown.

This fix is required for the integration tests in the following Celery pull request to pass successfully: celery/celery#9986

Fixes #434

[Copilot]

Pull Request Overview

This PR fixes an issue where task results could be lost during pool shutdown by ensuring the ResultHandler thread completes processing queued results before terminating.

• Removed the premature result_handler.terminate() call in _terminate_pool() to allow natural shutdown
• Added comprehensive test coverage for the graceful shutdown behavior with queued results
• Added clear documentation explaining the shutdown mechanism

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
billiard/pool.py	Removed result_handler.terminate() call and added documentation explaining that the ResultHandler thread will exit naturally after processing all queued results during shutdown
t/unit/test_pool.py	Added simple_task helper function and test_graceful_shutdown_delivers_results test to verify that results are properly delivered when _terminate_pool() is called with pending results in the queue

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[Nusnus]

Thank you!