test: add unit tests for safeCompare timing-safe comparison

[Sarthak-Nayak]

Closes #827

Summary of Changes:

• Exported safeCompare from src/app/api/webhooks/github/route.ts to make it accessible for the test runner.
• Added a comprehensive Vitest unit test suite covering:
  • Immediate false returns on length mismatch (before timingSafeEqual).
  • Correct true evaluations on identical strings.
  • Safe handling of empty strings without exceptions.
• Integrated a mock/spy on crypto.timingSafeEqual to actively verify that the timing-safe comparison logic is triggered appropriately.

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[Priyanshu-byte-coder]

Hi! This PR now has a merge conflict with main, likely on the shared e2e test files (e2e/dashboard-widgets.spec.js, e2e/auth-bypass.spec.js, e2e/landing.spec.js). Multiple PRs were merged recently and the e2e files updated. Please rebase onto the latest main:

git fetch origin
git rebase origin/main
# resolve any conflicts
git push --force-with-lease

Your actual feature/fix changes are valuable — we'd love to get this merged once rebased!