http: cache maxHeaderPairs per header section

[GetThatCookie]

This PR is one of four focused changes split out of #64980 following review
feedback.

The full context, rationale, related work, benchmarks, and validation details
are documented there.

The AI-assistance disclosure in #64980 applies to this split PR as well.

[nodejs-github-bot]

Review requested:

• @nodejs/http
• @nodejs/net
• @nodejs/performance

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.32%. Comparing base (f00fb75) to head (ef24255).
⚠️ Report is 39 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #64988      +/-   ##
==========================================
+ Coverage   90.27%   90.32%   +0.04%     
==========================================
  Files         762      759       -3     
  Lines      247534   247636     +102     
  Branches    46694    46693       -1     
==========================================
+ Hits       223457   223665     +208     
+ Misses      15529    15454      -75     
+ Partials     8548     8517      -31     

Files with missing lines	Coverage Δ
src/node_http_parser.cc	85.62% <100.00%> (+0.64%)	⬆️

... and 59 files with indirect coverage changes

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[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/75457/

[pimterry]

@GetThatCookie why the force push? Can you explain why the implementation has changed and that test code was dropped? It's helpful to provide context for reviewers when substantially changing a PR like this.

At a glance, it looks like this new code is just doing the same thing but in a more fragile less clear way, and with less testing? Presumably I'm missing something.