test(scripts): add tests for prepare-release script

Add test suite for release preparation script, validating version updates,
workflow steps, and diff generation against reference files.
Issue: DX-1201

Co-authored-by: llm-git <[email protected]>
Co-authored-by: Copilot Autofix powered by AI <62310815+github-advanced-security[bot]@users.noreply.github.com>

Author: 3 people