[MQTT] Test publish/subscribe end-to-end

[ezehf2]

Write two scripts — one publisher and one subscriber. The publisher sends a dummy JSON message to a topic defined in config.json. The subscriber listens to that same topic and prints the received message to the console. Run both at the same time and confirm the subscriber receives the publisher's message. Take a screenshot of the working output and attach it to this issue before closing. This is the sign-off test for Sprint 1 broker work.