Create new state property for alarm_control_panel which is using an enum

[gjohansson-ST]

Background

Some entity models (like alarm_control_panel) allows integration to directly overwrite state property to set it's state using constants and are not modeled like the newer ones (as example `lawn_mower) which has a separate property for handling state using an enum.

Proposal

Similar to the lawn_mower integration implement a separate state property alarm_state which is using an enum AlarmControlPanelEntityState for controlling it's state.

@final
@cached_property
def state(self) -> str | None:
    """Return the current state."""
    return alarm_state

@cached_property
def alarm_state(self) -> AlarmControlPanelEntityState | None:
    """Return the current alarm control panel entity state.

    Integrations should overwrite this or use the 'attr_alarm_state'
    attribute to set the alarm status using the 'AlarmControlPanelEntityState' enum.
    """
    return self._attr_alarm_state

After the deprecation period we would change property to be @final and integrations should only implement alarm_state returning AlarmControlPanelEntityState.

Draft conceptual PR:home-assistant/core#126283

Update after initial proposal

• Return the enum member instead of stringifying it's state

[MartinHjelmare]

The @final decorator is only used by type checkers, right? It won't break any runtime code? Could we add it already at the start of the deprecation period (after all core integrations have been migrated)?

[gjohansson-ST]

We can do that. It's only type checking.
I intended to run a race to fix all core integrations hence in the proposal the deprecation notice should only be for custom components.

[MartinHjelmare]

We've discussed this proposal in the Core team and decided to approve it.

[frenck]

Implemented in home-assistant/developers.home-assistant#2361