fix(http/unstable): sync HEADER const with IANA registry

[tomas-zijdemans]

• sync HEADER in unstable_header.ts with the IANA HTTP Field Name Registry (last updated 2025-12-02).
• doc fixes
• updated tests

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.21%. Comparing base (1cd85bf) to head (1aebf7a).
⚠️ Report is 7 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6991   +/-   ##
=======================================
  Coverage   94.20%   94.21%           
=======================================
  Files         613      613           
  Lines       47710    47726   +16     
  Branches     8299     8299           
=======================================
+ Hits        44945    44963   +18     
+ Misses       2697     2696    -1     
+ Partials       68       67    -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.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[tomas-zijdemans]

Closing due to: #7004

See discussion here