chore(ci): upgrade checkout to v6

[rejected-l]

Updates actions/checkout to v6 in CI workflows. No code changes.

https://github.com/actions/checkout/releases/tag/v6.0.0

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.90%. Comparing base (fc2e787) to head (2b42d48).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #839   +/-   ##
=======================================
  Coverage   99.90%   99.90%           
=======================================
  Files          26       26           
  Lines        1084     1084           
=======================================
  Hits         1083     1083           
  Misses          1        1           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

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