test: add Headers coverage and benchmark

[anonrig]

Adds Node-side coverage for the WHATWG Headers API and a fetch/headers benchmark.

These tests are not from WPT. They cover constructor init, get/set/append/delete, set-cookie / getSetCookie(), iteration (including mutation during iteration), validation, brand checks, and util.inspect.

The implementation change lives in undici:

Refs: nodejs/undici#5699

[nodejs-github-bot]

Review requested:

• @nodejs/performance
• @nodejs/security-wg

[jasnell]

AI agents are not permitted to use the Signed-off-by

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.14%. Comparing base (f83e7df) to head (2edacb0).
⚠️ Report is 48 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #65365      +/-   ##
==========================================
- Coverage   90.31%   90.14%   -0.18%     
==========================================
  Files         751      752       +1     
  Lines      249956   251884    +1928     
  Branches    47204    47379     +175     
==========================================
+ Hits       225745   227054    +1309     
- Misses      15612    16147     +535     
- Partials     8599     8683      +84     

see 98 files with indirect coverage changes

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[mcollina]

Code LGTM. Commit metadata needs fixing.

[anonrig]

Addressed the review feedback:

• Removed the AI Signed-off-by. The commit is now authored and signed off by Yagiz Nizipli.
• Fixed the quote-props lint failures in benchmark/fetch/headers.js and test/parallel/test-whatwg-headers.js.