Fix version bump baseline detection

[MaxGhenis]

Summary

• bump the release workflow from the highest known released version instead of trusting a stale pyproject version
• fetch tags in the versioning job so published tags are visible to the bump script
• add regression tests for stale pyproject and tag-aware version selection

Testing

• uv run pytest --noconftest tests/test_bump_version.py
• uv run ruff check .github/bump_version.py tests/test_bump_version.py