What and why?

Release PRs created by yarn version:all only bump the "version" field in every package.json. Because packages/base/package.json is in the path filters for the serverless e2e check jobs, those tests were always triggered on release commits even though no serverless code changed.

Example: https://github.com/DataDog/datadog-ci/actions/runs/26580292541/job/78312089121?pr=2334

How?

Adds a check-version-bump-only job that:

1. Fails fast if any non-package.json file was touched.
2. Greps the actual diff content -- if every +/- line matches "version": "X.Y.Z", the commit is version-bump-only.

The e2e-test job's SKIP_*_TESTS env vars now also skip when check-version-bump-only returns true.

Manual testing

Tested the script logic against two real branches:

alejandro.fontan/release-v5.18.0 (16 package.json files, version fields only):

non_json: ''
leftover lines: (none)
result=true  ✓

PR #2333 (tal.usvyatsky/datadog-ci-lambda-e2e-tests, real code changes):

non_json: '.github/CODEOWNERS .github/workflows/ci.yml e2e/fixtures/lambda/...'
result=false  ✓

Review checklist

• Feature or bugfix MUST have appropriate tests (unit, integration)