Normalize list-style header parsing in pure-Python parser

[rodrigobnogueira]

What do these changes do?

This PR fixes list-style token parsing for the Connection header and related tokenized headers in the HTTP parser.

The parser now normalizes comma-separated values and validates token fragments more consistently, avoiding edge-case misparsing.

Are there changes in behavior for the user?

Yes.

Malformed Transfer-Encoding values are now rejected more strictly during parsing.

Specifically, when parsing yields no valid transfer-coding tokens (for example because the field value is empty/invalid after normalization), the parser raises BadHttpMessage("Request has invalid \Transfer-Encoding`")`.

Previously, some malformed cases could be treated as "not chunked" and continue. The new behavior fails fast for invalid Transfer-Encoding, which is safer and more RFC-aligned.

Is it a substantial burden for the maintainers to support this?

No.

This is a focused parser correctness hardening with tests and changelog updates, and it keeps behavior consistent around invalid tokenized headers.

Related issue number

N/A

Checklist

• I think this is a desirable change and covered by tests.
• I have considered backward compatibility.
• I have considered possible performance impact.
• I have updated the documentation, or no docs update is needed.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.10%. Comparing base (2602b71) to head (ea80eb3).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #12253      +/-   ##
==========================================
- Coverage   99.11%   99.10%   -0.01%     
==========================================
  Files         130      130              
  Lines       45432    45489      +57     
  Branches     2400     2410      +10     
==========================================
+ Hits        45028    45084      +56     
  Misses        273      273              
- Partials      131      132       +1     

Flag	Coverage Δ
CI-GHA	98.96% <100.00%> (-0.01%)	⬇️
OS-Linux	98.70% <100.00%> (+<0.01%)	⬆️
OS-Windows	96.98% <100.00%> (+<0.01%)	⬆️
OS-macOS	97.86% <100.00%> (+<0.01%)	⬆️
Py-3.10.11	97.41% <100.00%> (+<0.01%)	⬆️
Py-3.10.20	97.89% <100.00%> (-0.01%)	⬇️
Py-3.11.15	98.09% <100.00%> (+<0.01%)	⬆️
Py-3.11.9	97.62% <100.00%> (+<0.01%)	⬆️
Py-3.12.10	97.71% <100.00%> (+<0.01%)	⬆️
Py-3.12.13	98.18% <100.00%> (-0.01%)	⬇️
Py-3.13.12	98.43% <100.00%> (-0.01%)	⬇️
Py-3.14.3	98.49% <100.00%> (+<0.01%)	⬆️
Py-3.14.3t	97.49% <100.00%> (+<0.01%)	⬆️
Py-pypy3.11.13-7.3.20	97.53% <100.00%> (+<0.01%)	⬆️
VM-macos	97.86% <100.00%> (+<0.01%)	⬆️
VM-ubuntu	98.70% <100.00%> (+<0.01%)	⬆️
VM-windows	96.98% <100.00%> (+<0.01%)	⬆️

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[codspeed-hq]

Merging this PR will not alter performance

✅ 59 untouched benchmarks

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_{Comparing rodrigobnogueira:fix-header-list-parsing-master (ea80eb3) with master (2602b71)}