Validate headers specific to upgrade request only when response is 101.

[tarasko]

No description provided.

[codspeed]

Merging this PR will improve performance by 14.07%

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⚠️ Different runtime environments detected

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⚡ 1 improved benchmark
✅ 2 untouched benchmarks

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
⚡	test_bench_echo[32768]	2 s	1.7 s	+14.07%

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---

_{Comparing custom_http_responses (3254076) with master (bbea420)}

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.78%. Comparing base (bbea420) to head (3254076).
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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #113      +/-   ##
==========================================
+ Coverage   91.00%   91.78%   +0.78%     
==========================================
  Files          18       18              
  Lines        2590     2667      +77     
  Branches      138      139       +1     
==========================================
+ Hits         2357     2448      +91     
+ Misses        190      176      -14     
  Partials       43       43              

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[tarasko]

The parser now enforces:

• GET, non-empty target, and exactly HTTP/1.1.
• Valid HTTP-token header names without stripping whitespace.
• Valid field values, OWS trimming, and no obsolete folding.
• Maximum 128 headers.
• No Transfer-Encoding.
• At most one Content-Length, whose value must represent zero.
• Existing 16 KiB total header limit remains.
• Expect remains accepted and unprocessed.

WebSocket validation has direct unit coverage for every existing branch, including accept-value calculation and non-ASCII keys.

[tarasko]

Closes #112