version-bump-check: test app metadata checks and the git tag fallback

[mairas]

tests/version-bump-check.test.sh (added in PR 63) covers only the repository-level VERSION check against a stubbed gh release list. Found by review there, untested paths in .github/workflows/version-bump-check.yml:

• Part A: a change under apps/<name>/ without apps/<name>/metadata.yaml fails; with it, passes. This is the main check in the three container repositories.
• The fallback to git tag -l 'v*' when gh release list fails.
• The skip when no stable release tag exists.
• The failure message listing only the package-affecting files when a lockfile and a source file change together.

The step-test harness in tests/lib/step.sh supports all of these (stub_gh honours GH_EXIT).