Add MQTT and timer manager unit tests

[AndreasBeyer]

Summary

• add a TimerManager unit test module that exercises timer registration, cancellation, clearing, and lock usage
• add an MQTT unit test module that mocks the paho client and verifies start/stop behavior, reconnect backoff, and all publish helpers
• update the custom test runner to add the repository root to sys.path so that the new tests are discoverable and import project modules correctly

Testing

• python tests/run_tests.py

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Codex Task