Fix OptionsFlow config_entry attribute error for HA 2025.12+

[f1egmatik]

Breaking change

Proposed change

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New feature (which adds functionality to an this integration)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Example entry for configuration.yaml:

# Example configuration.yaml

Additional information

• This PR fixes or closes issue: fixes stop workint in HA 2025.12 #230
• This PR is related to issue:

Removed self.config_entry = config_entry from GismeteoOptionsFlowHandler.__init__ as it's no longer allowed to be set manually in Home Assistant Core 2025.12+.

Checklist

• The code change is tested and works locally.
• There is no commented out code in this PR.
• The code has been formatted using Black (black --fast custom_components)

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated to README.md