chore: pin GitHub Actions workflows to full commit SHAs

[irfanuddinahmad]

Pins all GitHub Actions uses: references to their full commit SHA values, with version tags preserved as inline comments for readability.

Part of org-wide security hardening initiative (openedx/.github#165) to prevent supply chain attacks via mutable action version tags.

Note: This PR replaces #498 which was created from a fork and could not access org secrets for NPM authentication. This PR is from an org branch and should pass CI correctly.

Supersedes #498

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.89%. Comparing base (3b74bb7) to head (9ec9494).
⚠️ Report is 20 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #499   +/-   ##
=======================================
  Coverage   78.89%   78.89%           
=======================================
  Files          37       37           
  Lines         758      758           
  Branches      194      194           
=======================================
  Hits          598      598           
  Misses        147      147           
  Partials       13       13           

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