#55: Wait on MQTT publish queue instead of sleeping

[Aydogan Ersoz (silabs-aydogane)]

Issue

• MQTT publish can wait up to 100 ms before being sent #55
• JIRA: NOJIRA

Closes: #55

Change

• Wait on the MQTT publish queue instead of sleeping when it is empty. publish_queue.pop() returns as soon as a message is pushed.
• Drain the other MQTT queues (subscribe, unsubscribe, unretain, reset) with non-blocking try_pop().

Checklist

• Pull request template has been filled out correctly
• ./ci/scripts/clang-tidy.sh check has been run locally (see CONTRIBUTING.md)
• Documentation has been updated if necessary
• A Contribution License Agreement has been established between Silicon Labs (@SiliconLabs) and author's company (matching email domain)

[cursor]

PR Summary

Medium Risk
Changes timing and scheduling on the MQTT handler thread, which serializes all broker I/O; publish latency should improve but subscribe/unretain handling now interleaves differently with blocking publish waits.

Overview
Refactors mqtt_handler::run() so outbound publishes wake the handler promptly instead of waiting for a fixed poll/sleep cycle.

Non-publish work (reset, subscribe, unsubscribe, unretain) is still drained each iteration with non-blocking try_pop(). The loop then blocks on publish_queue.pop() for up to mqtt_client_poll_interval, which returns as soon as a publish is queued, processes that message, and bursts any additional queued publishes with try_pop(). The old processed_any flag and unconditional sleep when every queue was empty are removed in favor of this timed wait on the publish queue.

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