ci: reenable shared builtin CI tests

[mcollina]

Summary

Re-enable the shared builtin CI workflow for supported/current Node.js majors.

Changes

• run the shared builtin CI job for Node.js 24 and 25 again
• include Node.js 26 so the reusable workflow will start testing it automatically once a release is available
• keep Node.js 20 and 22 excluded, with a note that they still hit upstream Node.js issues with --shared-builtin-undici/undici-path

Notes

The reusable workflow already skips unreleased versions cleanly, so including Node.js 26 here is safe.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.96%. Comparing base (1694d12) to head (615df31).
⚠️ Report is 1 commits behind head on fix/websocket-http1-fallback.

Additional details and impacted files

@@                      Coverage Diff                      @@
##           fix/websocket-http1-fallback    #4967   +/-   ##
=============================================================
  Coverage                         92.96%   92.96%           
=============================================================
  Files                               111      111           
  Lines                             35856    35856           
=============================================================
  Hits                              33334    33334           
  Misses                             2522     2522           

☔ 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.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.