test(ci): update release workflow tests for GitHub username lookup

Replace the old author attribution test with two targeted tests that
validate the GitHub API username resolution in both stable and
nightly changelog generation steps.

Add _extract_step_block helper to isolate individual workflow steps
for precise assertions. Each test verifies:

- GitHub commit API query per SHA
- jq extraction of author.login
- @username mention format for resolved users
- Plain author name fallback without @ prefix
- Absence of the old direct git author formatting