LuxPower Freeze Charging support: documentation updates, template improvements, and warning suppression

[brickatius]

PR Description

This PR improves support for LuxPower inverters when using Predbat Freeze Charging and Freeze Export, focusing on documentation clarity, configuration correctness, and stable runtime behaviour.

Summary of changes

Documentation

• Revises the LuxPower section of Inverter Setup to fully document how Freeze Charging and Freeze Export are implemented for LuxPower inverters.

• Explains the required Home Assistant helpers and automations.

• Introduces and documents the new maintain_freeze_charge_status inverter option, including why it is required for LuxPower.

Configuration

Updates the luxpower.yaml example template to:

• Enable Freeze Charging / Freeze Export where supported.

• Reference battery_min_soc from the LuxPython integration to avoid discrepancies between user configuration and inverter state.

• Reference battery_rate_max from an input_number helper rather than a hard-coded value.

• Adds maintain_freeze_charge_status to the inverter configuration options and documents its purpose.

Runtime behaviour

• Updates inverter.py to support maintain_freeze_charge_status.

• When enabled, Predbat suppresses expected warnings caused by manipulating scheduled_charge_enable during Freeze Charging.

• This prevents predbat.status from transitioning into Warn: states during normal Freeze Charging operation, allowing existing automations (e.g. Freeze exit handling) to behave reliably.

• Suppression only applies during Freeze Charging and does not affect normal charging behaviour or genuine error conditions.

Rationale

• LuxPower inverters do not provide native Freeze Charging support and rely on controlled manipulation of scheduled_charge_enable. During Freeze Charging, this can legitimately cause Predbat to observe a mismatch between expected and reported state, resulting in warnings that are expected but disruptive.

• maintain_freeze_charge_status ensures Predbat treats these cases as expected behaviour only while Freeze Charging is active, keeping predbat.status stable and avoiding unintended automation triggers.

Notes

No functional changes to Predbat planning logic.

Changes are opt-in and limited to inverters that enable maintain_freeze_charge_status.

Fixes #2916