test: guard release notes fallback behavior

[X-PG13]

Summary

• make the release workflow fallback notes point maintainers to the missing release notes file
• add a release metadata test that guards the fallback contract

Validation

• ./.venv-dev/bin/python -m unittest tests.test_release_metadata -v
• git diff --check
• make check PYTHON=./.venv-dev/bin/python

Closes #134