fix: MQTT callbacks after transport garbage collection

[Carter Tinney (cartertinney)]

Summary

• stop Paho's network loop when a callback fires after its owning MQTTTransport has been garbage-collected
• guard every callback installed by _create_mqtt_client (on_connect, on_disconnect, on_subscribe, on_unsubscribe, on_publish, and on_message) before dereferencing the transport
• add focused garbage-collection regression coverage, including both successful and failed CONNACK paths and loop-cleanup assertions

Fixes #1192.

This supersedes #1193. That PR guarded only the successful on_connect path, placed its check after the failed-CONNACK branch, and logged that the loop was stopping without calling loop_stop(). This change uses one cleanup path for all six callbacks, actually stops the Paho loop, and preserves propagation of cleanup errors.

Validation

• python -m pytest tests/unit/common/test_mqtt_transport.py — 281 passed
• python -m pytest tests/unit — 5,443 passed
• Black 22.3.0 check on changed files
• Flake8 7.0.0 check on changed files

[Copilot]

Pull request overview

Prevents MQTT callback failures after their owning MQTTTransport is garbage-collected.

Changes:

• Centralizes weak-reference validation and Paho loop shutdown.
• Guards all six MQTT callbacks.
• Adds garbage-collection regression tests.

Reviewed changes

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

File	Description
azure-iot-device/azure/iot/device/common/mqtt_transport.py	Safely stops callbacks after transport collection.
tests/unit/common/test_mqtt_transport.py	Covers callback cleanup across all callback types.

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