ci: resolve the subrelease tags

[chaithubk]

The failure was caused by resolving the first tag from an empty [grouped_release_tags], which produced an empty string and crashed git rev-list.

• Updated the commit-resolution step in combine-release.yml:
  • If grouped_release_tags is empty, it now falls back to:
    • core-image-minimal-<release_version>
    • core-image-medtech-<release_version>
  • Resolves the first tag commit via GitHub API (not local git), so it works even with shallow checkout.
  • Handles annotated tags correctly by dereferencing tag objects to commit SHA.
  • Emits a clear error if no first tag can be determined.
• Cleaned link formatting in generate-combined-release.py:
  • Bundled releases now render as clean list items (links only), matching your preferred format.