Support unloading the config entry

This is a Silver rule. In quality_scale.yaml, it is called config-entry-unloading.

The integration must implement async_unload_entry in its __init__.py. When a config entry is unloaded, the integration unloads its platforms and cleans up everything it set up: listeners, subscriptions, background tasks, and open connections.

Unloading is what makes it possible to reload, disable, or remove an integration while Home Assistant keeps running. Without it, the user has to restart Home Assistant to apply a change or get rid of the integration. And whatever is not cleaned up keeps running in the background, using memory and connections for nothing.

Unload the platforms, and clean up when that succeeds:

async def async_unload_entry(
    hass: HomeAssistant, entry: MyConfigEntry
) -> bool:
    """Unload a config entry."""
    if unload_ok := await hass.config_entries.async_unload_platforms(
        entry, PLATFORMS
    ):
        await entry.runtime_data.client.disconnect()

    return unload_ok

Often, you do not need to keep track of the cleanup yourself. Register it during setup with entry.async_on_unload, and Home Assistant calls it for you:

async def async_setup_entry(
    hass: HomeAssistant, entry: MyConfigEntry
) -> bool:
    """Set up My integration from a config entry."""
    client = MyClient(entry.data[CONF_HOST])
    entry.async_on_unload(client.subscribe(_handle_event))
    ...

The callbacks registered with entry.async_on_unload run when async_unload_entry succeeds, and also whenever async_setup_entry fails: when it raises, like ConfigEntryNotReady, ConfigEntryAuthFailed, or ConfigEntryError, or when it returns False. You still need async_unload_entry itself: without it, the entry cannot be unloaded at all.

• Pylint, when the rule is done: __init__.py must define async_unload_entry (home-assistant-missing-config-entry-unloading).
• Code that tests never run shows up in the test coverage, so a test that unloads the entry is usually needed anyway.
• Whether everything is cleaned up is checked in review.

There are no exceptions to this rule.

• entity-event-setup: subscribe to events in the entity lifecycle.