Consume MQTT ack futures on abandonment to prevent asyncio warnings

[eman]

Fixes #97

Problem

_await_ack() in mqtt/connection.py (used by connect/disconnect/close/subscribe/unsubscribe/publish) and the inline acknowledgement waits in mqtt/subscriptions.py's subscribe()/unsubscribe() shield the AWS CRT future so a timeout or task cancellation never propagates into the SDK future — the underlying operation keeps running in the background.

If that shielded future later completes with an exception (notably AwsCrtError / AWS_ERROR_MQTT_CANCELLED_FOR_CLEAN_SESSION during a reconnect race) after the awaiting task has already given up, nobody ever retrieves the exception. asyncio then logs an unhandled Future exception was never retrieved / exception in shielded future warning at garbage-collection time, which is exactly the noise that the Home Assistant integration had to suppress with a global loop exception handler.

Fix

Attach a done_callback to the shielded future whenever a wait is abandoned (cancelled or timed out) so its eventual result/exception is always retrieved. Benign errors are logged at debug level instead of leaking as an unhandled asyncio warning.

• mqtt/connection.py: new _consume_abandoned_future() helper, wired into both the CancelledError and TimeoutError branches of _await_ack().
• mqtt/subscriptions.py: same pattern applied to the inline subscribe()/unsubscribe() waits (also added the previously-missing TimeoutError handling/logging there).

Testing

• Added TestAbandonedAckFutureConsumed regression tests in tests/test_mqtt_reliability.py covering both the timeout and cancellation paths. Verified these tests fail (asyncio logs "exception in shielded future") without the fix and pass with it.
• ruff check/ruff format --check: pass
• mypy src/nwp500: no issues
• pytest: 611/611 passed

[Copilot]

Pull request overview

This PR prevents noisy asyncio warnings caused by MQTT acknowledgement futures that outlive their awaiting task (timeout/cancellation), by ensuring the eventual result/exception is still retrieved and logged instead of being left for GC to report as “Future exception was never retrieved”.

Changes:

• Add _consume_abandoned_future() and wire it into _await_ack() cancellation/timeout paths to consume late exceptions.
• Apply the same “consume abandoned ack future” pattern to subscribe()/unsubscribe() acknowledgement waits (and add missing timeout handling there).
• Add regression tests covering both timeout and cancellation abandonment scenarios, and document the fix in the changelog.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 4 comments.

File	Description
src/nwp500/mqtt/connection.py	Adds consumption of late ack future exceptions when _await_ack() is abandoned.
src/nwp500/mqtt/subscriptions.py	Extends the abandonment-consumption pattern to subscribe/unsubscribe ack waits and adds timeout handling.
tests/test_mqtt_reliability.py	Adds regression tests to ensure abandoned ack futures don’t leak asyncio warnings.
CHANGELOG.rst	Documents the fix under Unreleased.