mqtt: polish published state values

[filiplajszczak]

Publishes SoC with decimal precision, normalizes discharge-blocked state to lowercase booleans, fixes its HA discovery template, and avoids duplicate MQTT subscriptions on reconnect.

[Copilot]

Pull request overview

This PR polishes MQTT state publishing to better align with Home Assistant expectations and reconnect behavior in batcontrol’s MQTT integration.

Changes:

• Publish SOC with decimal precision (2 decimals) instead of zero-padded integers.
• Normalize discharge_blocked published payloads to lowercase (true/false) and fix its HA discovery value_template.
• Fix reconnect subscription logic to avoid duplicate subscriptions, and add regression tests for these behaviors.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/batcontrol/mqtt_api.py	Fixes reconnect subscribe loop, improves SOC formatting, normalizes discharge_blocked payloads, and corrects HA discovery template logic.
tests/batcontrol/test_mqtt_api.py	Adds tests covering reconnect subscription behavior and published state payload formatting / discovery template expectations.