test(mqtt): add unit tests for MQTTRepositoryImpl

Cover constructor, disconnect(), publish() drop when client is null,
address parsing with TLS fallback port, and exponential backoff logic.

Reconnect loop and publishReceived callbacks require an integration test
with an embedded MQTT broker (e.g. Moquette) as MQTTClient.run() is a
blocking call that needs a real TCP connection — out of scope for unit tests.

Author: theKorzh