When the live adapter delivery path (_deliver_result) or media send path
(_send_media_via_adapter) times out at future.result(timeout=N), the
underlying coroutine scheduled via asyncio.run_coroutine_threadsafe can
still complete on the event loop, causing a duplicate send after the
standalone fallback runs.
Cancel the future on TimeoutError before re-raising, so the standalone
fallback is the sole delivery path.
Adds TestDeliverResultTimeoutCancelsFuture and
TestSendMediaTimeoutCancelsFuture.
18e7fd8364