Fix: avoid assigning to read-only config_entry in OptionsFlow (fixes options UI 500 error)

[jelofsson]

This fixes a 500 error when opening the Greenely integration Options (Configure) UI.

Background:
Historically integrations could set self.config_entry (though it was discouraged and logged a deprecation warning). Beginning with Home Assistant Core 2025.12 the deprecation period ends, self.config_entry becomes truly read-only and attempting to assign to it raises an AttributeError (which produces the 500 error shown in the UI).

What I changed:

• Store the passed ConfigEntry in a private attribute instead:
  • self._config_entry = config_entry
• Use self._config_entry.options.get(...) to populate option defaults in the options schema.
• Corrected the type hint in async_get_options_flow to accept a ConfigEntry.

Tested:

• Restarted Home Assistant and verified the Options (Configure) form opens without the AttributeError / 500 error.
• Verified option defaults load correctly from the stored config entry, And that changes are saved and loaded correctly.