fix(mqtt_publisher): pass tls_insecure=None for non-TLS connections

[csutcliff]

Problem

Since the aiomqtt migration (#457), the gateway hangs at startup when configured with a plain tcp:// broker URI. It logs

[INFO]: Connecting to MQTT Broker - mqtt_gateway
[INFO]: MQTT client disconnected - publisher.mqtt_publisher

and then does nothing forever: no connection ever reaches the broker, no error is logged, no retry happens, and the process stays alive. 0.12.0 works with the same configuration.

Root cause

MqttPublisher.__run_loop passes tls_insecure=bool(ssl_context and ...) to aiomqtt.Client. Without TLS that evaluates to False rather than None, and aiomqtt applies any non-None value via paho's tls_insecure_set(), which raises:

ValueError: Must configure SSL context before using tls_insecure_set.

The ValueError isn't one of the exceptions the reconnect loop handles (MqttConnectError/MqttError/CancelledError), so the connect task dies with only the finally block's "MQTT client disconnected" log, the connected event is never set, and connect() waits on it forever.

Fix

Only pass a tls_insecure value when TLS is enabled with hostname checking disabled; otherwise leave it None so aiomqtt skips tls_insecure_set() entirely. Semantics for the TLS cases are unchanged.

Added regression tests covering the three combinations (plain TCP → None, TLS without hostname check → True, TLS with hostname check → None); the plain-TCP and TLS-with-check cases fail against current develop. mypy, ruff check, ruff format --check and pytest all pass.