Clarify workflow YAML tooling warning

[mfoltz]

Motivation

• Make the contributor-facing review/testing note accurately describe the local failure mode observed when validating .github/workflows/release.yml, and avoid wording that implies Python itself is unavailable or broken.

Description

• Add a ## Review/testing note to .github/CONTRIBUTING.md that preserves the python3 check, documents the exact failure ModuleNotFoundError: No module named 'yaml', and clarifies this is a missing PyYAML/tooling dependency rather than evidence that .github/workflows/release.yml is invalid YAML.

Testing

• Verified the updated note and file content with sed -n '1,120p' .github/CONTRIBUTING.md and reproduced the failure using python3 - <<'PY'\nimport yaml\nPY which fails with ModuleNotFoundError: No module named 'yaml', matching the documented behavior.

---

Codex Task

[Copilot]

Pull request overview

Updates contributor documentation to clarify that a common local “YAML validation” warning for .github/workflows/release.yml can stem from missing Python tooling (PyYAML), not from invalid workflow YAML.

Changes:

• Add a “Review/testing note” to .github/CONTRIBUTING.md.
• Document the exact local failure mode: ModuleNotFoundError: No module named 'yaml' from import yaml.
• Clarify this is an environment/tooling dependency issue (missing PyYAML), not proof the workflow file is invalid.