Feature: Test MQTT Connection for ESP Device

[maitiSoutrik]

Add a new MCP tool test_mqtt_connection(broker_url: str, port: int, username: str, password: str) to validate MQTT broker connectivity.

Functionality:

• Takes MQTT broker details (URL, port, optional username/password) as input.
• Attempts to connect to the MQTT broker.
• Publishes a test message to a topic and subscribes to confirm receipt (optional).
• Returns connection status and any errors.