fix(defer): propagate CancelledError into background event loop

[no0ktheali3n]

DeferredTask.result() awaits on loop.run_in_executor(None, _get_result), where _get_result is a blocking call to self._future.result(timeout). When the outer asyncio task is cancelled (for example a wall-clock timeout in the caller fires), only the outer task receives the CancelledError. The underlying concurrent.futures.Future continues running on the singleton background event loop, and the thread-pool worker stays parked in _get_result waiting for it. Every cancelled result() leaks one worker thread until the default ThreadPoolExecutor saturates and subsequent result() calls block indefinitely.

Fix: catch asyncio.CancelledError in result(), call self.kill() to cancel the underlying future and drain the singleton background event loop, then re-raise. The worker thread is released as the future resolves to CancelledError; any in-flight coroutine tasks on the background loop are cancelled too.

Adds tests/test_defer_cancellation.py asserting that after a cancelled result(), the underlying future reaches done() within 500ms

• a thread-leak regression guard.

Reproduces in upstream main and earlier releases - same code shape.