Add GitHub Action SHA validation to CI

[cjimti]

Description

Adds a script and CI job to validate that all SHA-pinned GitHub Actions reference valid commits. Prevents CI failures from typos in SHA hashes.

Changes:

• scripts/validate-action-shas.sh - Parses workflow files and validates SHAs via GitHub API
• New validate-actions CI job that runs on every push/PR

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• Test improvement (new or updated tests)
• Documentation update
• Stability/performance improvement
• Build/CI improvement

Note: New features are developed by maintainers only. See CONTRIBUTING.md for details.

Related Issues

Prevents issues like PR #308 where an invalid SHA caused CI failures.

Testing

• Ran go test ./... locally
• Tested manually with a Kubernetes cluster
• Added new tests for changes (if applicable)

Script validation:

$ ./scripts/validate-action-shas.sh
Validating GitHub Action SHA pins...

✓ actions/checkout@11bd71901... (.github/workflows/ci.yml)
✓ actions/setup-go@3041bf56c... (.github/workflows/ci.yml)
...

All SHA pins are valid

Checklist

• My code follows the project's style guidelines (go fmt, go vet)
• I have read CONTRIBUTING.md
• I have updated documentation if needed
• This PR is focused and does not include unrelated changes

Screenshots/Logs (if applicable)

Example failure output (invalid SHA):

✗ codecov/codecov-action@1e68e06... - SHA not found! (.github/workflows/ci.yml)

Found 1 invalid SHA(s)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 59.42%. Comparing base (0174c41) to head (4e36051).
⚠️ Report is 4 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #321   +/-   ##
=======================================
  Coverage   59.42%   59.42%           
=======================================
  Files          38       38           
  Lines        4318     4318           
=======================================
  Hits         2566     2566           
  Misses       1620     1620           
  Partials      132      132           

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.