Normalize multiple Transfer-Encoding headers before chunked detection

[jmestwa-coder]

Summary

Normalize multiple Transfer-Encoding headers into a single comma-separated value before chunked detection.

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Motivation

RFC 9110 §5.3 defines that multiple header fields are equivalent to a single comma-separated value. This makes the parser behavior explicit and consistent with the RFC.

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Changes

• Use headers.getall(...) for Transfer-Encoding
• Join values before evaluating chunked encoding
• No change to existing parsing logic

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Test

Add test covering split Transfer-Encoding headers (gzip + chunked) and verifying correct parsing of multiple responses.

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What do these changes do?

Normalize multiple Transfer-Encoding headers into a single comma-separated value before evaluating chunked encoding, in line with RFC 9110 §5.3.

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Are there changes in behavior for the user?

No.
This makes header handling explicit and consistent without changing observable behavior.

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

No.
The change is small, self-contained, and limited to parsing normalization. It does not introduce new APIs or complexity.

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Related issue number

N/A

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Checklist

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes
• If you provide code modification, please add yourself to CONTRIBUTORS.txt
• Add a new news fragment into the CHANGES/ folder (.misc)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.92%. Comparing base (0471d65) to head (1998286).
⚠️ Report is 2 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #12465   +/-   ##
=======================================
  Coverage   98.92%   98.92%           
=======================================
  Files         134      134           
  Lines       46750    46759    +9     
  Branches     2429     2429           
=======================================
+ Hits        46248    46257    +9     
  Misses        373      373           
  Partials      129      129           

Flag	Coverage Δ
CI-GHA	98.98% <100.00%> (+<0.01%)	⬆️
OS-Linux	98.72% <100.00%> (-0.01%)	⬇️
OS-Windows	96.98% <100.00%> (+<0.01%)	⬆️
OS-macOS	97.89% <100.00%> (+<0.01%)	⬆️
Py-3.10.11	97.39% <100.00%> (+<0.01%)	⬆️
Py-3.10.20	97.86% <100.00%> (+<0.01%)	⬆️
Py-3.11.15	98.12% <100.00%> (+<0.01%)	⬆️
Py-3.11.9	97.65% <100.00%> (+<0.01%)	⬆️
Py-3.12.10	97.73% <100.00%> (+<0.01%)	⬆️
Py-3.12.13	98.20% <100.00%> (+<0.01%)	⬆️
Py-3.13.13	98.45% <100.00%> (+<0.01%)	⬆️
Py-3.14.4	98.50% <100.00%> (-0.01%)	⬇️
Py-3.14.4t	97.52% <100.00%> (+<0.01%)	⬆️
Py-pypy3.11.15-7.3.21	97.35% <100.00%> (+<0.01%)	⬆️
VM-macos	97.89% <100.00%> (+<0.01%)	⬆️
VM-ubuntu	98.72% <100.00%> (-0.01%)	⬇️
VM-windows	96.98% <100.00%> (+<0.01%)	⬆️
cython-coverage	38.09% <100.00%> (+0.01%)	⬆️

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[codspeed-hq]

Merging this PR will not alter performance

✅ 67 untouched benchmarks
⏩ 4 skipped benchmarks¹

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_{Comparing jmestwa-coder:normalize-transfer-encoding-headers (1998286) with master (0471d65)²}

Footnotes

1. 4 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

2. No successful run was found on master (0f0b0e5) during the generation of this report, so 0471d65 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩