Run Jest unit tests on every PR and push to main

[lornakelly]

Summary

Add a GitHub Actions workflow that executes Jest unit tests on every Pull Request and on pushes to main, blocking merges if any test fails. Test coverage reporting is out of scope for this issue.

Goals

• Automatically run all Jest unit tests on:
  • Every Pull Request
  • Pushes to main
  • Fail the CI workflow if any test fails to prevent merge
  • Keep configuration simple and focused (no coverage thresholds for now).

Non-Goals

• Enforcing code coverage thresholds.
• Running E2E tests as part of this workflow (separate issue).
• Complex test matrix across multiple Node versions (can be added later).
• Performance benchmarking or snapshot review policies.