fix: parse release-plz PR output

[P4suta]

Summary

• parse release-plz 0.3.160 output from the documented prs array
• treat an empty array as no release and reject multiple PRs because all product crates share one version group
• pin the JSON-shape contract in CI automation checks

Verification

• actionlint .github/workflows/release-pr.yml
• python3 tools/check_ci_contracts.py
• python3 -m unittest tools/test_release_metadata.py tools/test_publish_crates.py tools/test_sync_release_docs.py
• extraction checked against the exact JSON shape emitted by failed run 33224461887

Fixes the synchronization failure after release-plz successfully opened #18.