The new lazy initialization logic in `GetMessanger()` (lines 166-214) lacks test coverage. This includes critical paths like:

[rustyeddy]

The new lazy initialization logic in GetMessanger() (lines 166-214) lacks test coverage. This includes critical paths like:

1. Environment variable fallback logic (MQTT_BROKER, MQTT_USERNAME, MQTT_PASSWORD)
2. The "address already in use" error handling for the embedded broker
3. The config priority (config -> env vars -> default)
4. The fallthrough from "otto" case to default case

The existing TestGetMessanger only tests singleton behavior after NewMessanger is called, not the lazy initialization path. Consider adding table-driven tests that exercise these scenarios.

Originally posted by @Copilot in #34 (comment)