mqtt: publish control source state

[filiplajszczak]

Publishes a diagnostic MQTT state showing whether the current control state was last selected by the optimizer or through batcontrol's MQTT API.

[Copilot]

Pull request overview

This PR adds a new diagnostic MQTT state topic that indicates whether the currently applied control state was last selected by Batcontrol’s optimizer loop or via the MQTT API override path.

Changes:

• Add a new MQTT state topic /control_source plus Home Assistant MQTT discovery for a diagnostic sensor.
• Track and publish last_control_source from the core control path (optimizer-driven run() vs API-driven api_set_*() methods).
• Extend unit tests to cover discovery + publishing behavior and core publishing semantics.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
src/batcontrol/mqtt_api.py	Adds TOPIC_CONTROL_SOURCE, publish_control_source(), and HA discovery entry for the new diagnostic sensor.
src/batcontrol/core.py	Introduces control source constants, tracks last_control_source, and publishes it on optimizer/API actions and via refresh_static_values().
tests/batcontrol/test_mqtt_api.py	Adds tests ensuring HA discovery contains the new diagnostic sensor and that publish uses the correct topic.
tests/batcontrol/test_core.py	Adds tests verifying optimizer/API paths set and publish the expected control source.

[MaStr]

Just realized I need to publish "request change" to get my review published

[MaStr]

I had an if on the setter in my mind to just return of new = old