fix(mqtt): allow nonfatal advisory to start publisher

[TomGaussiran]

Summary

• Start the MQTT publisher when configuration validation returns a non-fatal advisory, matching MqttConfigError::is_fatal() and the existing MQTT example.
• Continue blocking publisher startup for fatal configuration errors.
• Add regression tests for valid, advisory, and fatal startup decisions.
• Run those binary tests in the MQTT workflow and trigger the workflow for src/main.rs changes.

Why

MqttConfig::validate() intentionally returns PlaintextOnPublicHost as a non-fatal advisory during the ADR-115 deprecation window. The sensing server treated every validation error as fatal, so a plaintext broker on another LAN host never started the publisher even though the error explicitly allowed startup.

Test plan

• MQTT startup regressions: 3 passed, 0 failed
• MQTT-enabled library suite: 486 passed, 0 failed, 1 ignored
• Mosquitto-backed integration suite: 3 passed, 0 failed
• MQTT-enabled sensing-server build
• Workflow YAML parsed successfully

No ESP32 firmware change is required.