Restrict theme asset downloads to .yaml files

[saikumarvasa100-hash]

Closes #4863

When installing themes with release assets containing .yaml files, HACS was downloading ALL release assets instead of only the theme .yaml files.

This fix adds a filter in the gather_files_to_download() method to skip non-.yaml files when downloading theme release assets. This ensures only .yaml theme files are downloaded, preventing unwanted files like zip archives from being installed.

[ludeeus]

FYI: This is not a fix, this is a breaking change.

Please also add coverage that ensure this do what you change here.

[saikumarvasa100-hash]

Thank you for the feedback! I understand this is a breaking change, not just a fix. I'm working on adding test coverage now to ensure the change works as intended. I'll update the PR shortly with tests that verify only .yaml files are downloaded for theme release assets.