Enhance the workflow validator to correctly handle GitHub Actions reusable workflows:
- Skip validation of 'runs-on' field for jobs with a 'uses' field
- Skip validation of the 'steps' field for reusable workflow jobs
- Add specific format validation for reusable workflow references
- Make workflow name optional for files that only contain reusable workflow jobs
- Add comprehensive test workflow files for various reusable workflow scenarios
This fixes validation errors when using GitHub's workflow_call feature that
allows reusing workflows from other repositories or local files.
