🧪 Add tests for MQTT client subscribe_registered

[manupawickramasinghe]

🎯 What: Added missing unit tests for RuViewMqttClient.subscribe_registered method.
📊 Coverage: Added tests to cover the happy path where _client.subscribe is called for all registered handlers after the client has been started, and an early exit branch where the function returns early and does nothing if called before the client is started.
✨ Result: Increased testing coverage for the MQTT client logic.

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PR created automatically by Jules for task 11411175522721673085 started by @manupawickramasinghe

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