mqtt: extract shared control topic helpers

[filiplajszczak]

This extracts shared MQTT control-topic helpers/constants to reduce repeated magic strings across publish and discovery paths.

[Copilot]

Pull request overview

This PR refactors MqttApi to centralize shared MQTT control-topic strings and topic construction logic, reducing duplicated topic concatenation across publish, subscribe, and Home Assistant discovery paths.

Changes:

• Introduced module-level MQTT topic constants and helper methods for building state and command topics.
• Updated publish/discovery/subscription code paths to use _topic() / _set_topic() instead of repeated string concatenation.
• Simplified MQTT discovery-related tests by extracting a shared discovery stub helper.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/batcontrol/mqtt_api.py	Adds shared topic constants and _topic() / _set_topic() helpers; updates relevant publish/discovery/subscribe call sites to use them.
tests/batcontrol/test_mqtt_api.py	Extracts _make_discovery_stub() to reduce repeated test setup for discovery-related assertions.