Start the 5.0 integration when it is installed next to 4.x

[RobHofmann]

Summary

Version 5.0 (branch 5.0-dev, domain gree_custom) moves a 4.x setup over when it starts: #519. But Home Assistant only sets up an integration that has a config entry or a YAML key, and after an update through HACS gree_custom has neither. HACS also leaves custom_components/gree in place, so this version still runs after the update.

This PR makes async_setup start gree_custom when it is installed. When it is not installed, nothing happens. Users who update to this release before they update to 5.0 then move over without doing anything. Users who skip it can still start the move by opening the setup flow of 5.0.

Testing

On the dev instance, with 5.0 installed next to this change:

• UI entry: gree_custom started, and the devices, entities and device row moved.
• gree: YAML block: gree_custom started, and the YAML was converted.
• With the original 4.0.6 code, gree_custom did not start. This confirms that the hook is needed.

With gree_custom not installed, async_get_integration raises IntegrationNotFound and the hook returns. Ruff with default rules reports nothing.

Notes

• Draft, because it only makes sense together with the 5.0 migration PR. It is safe to release before 5.0: it does nothing until gree_custom is installed.
• No version bump. That goes through the release workflow.