fix: decode multiple Content-Encoding codings in reverse order (RFC 9110)

[waterWang]

Summary

RFC 9110 §8.4 allows Content-Encoding to list multiple codings. The client must decode them in reverse order. When a server sends a body compressed twice with Content-Encoding: gzip,gzip, aiohttp currently does not decode the body because the header value "gzip,gzip" does not match the single-coding set {"gzip", "deflate", "br", "zstd"}.

Changes

1. HttpParser.parse_headers() — Parse Content-Encoding as a comma-separated list. Validate each coding individually and store the normalized list as a comma-separated string (e.g., "gzip,gzip").

2. HttpPayloadParser.__init__() — Chain multiple DeflateBuffer instances in reverse order so each coding is decoded in the correct sequence (last applied first).

3. DeflateBuffer.__init__() — Forward _low_water from the wrapped stream to support safe chaining of DeflateBuffer instances.

Testing

Expected: {"hello": "world"} when the endpoint sends Content-Encoding: gzip,gzip with a doubly-gzipped JSON body.

Fixes #13364

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.99%. Comparing base (b2b7b50) to head (5371160).
⚠️ Report is 32 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff            @@
##           master   #13377    +/-   ##
========================================
  Coverage   98.98%   98.99%            
========================================
  Files         132      132            
  Lines       49023    49297   +274     
  Branches     2551     2568    +17     
========================================
+ Hits        48526    48802   +276     
+ Misses        373      371     -2     
  Partials      124      124            

Flag	Coverage Δ
Autobahn	22.06% <9.09%> (-0.07%)	⬇️
CI-GHA	98.90% <100.00%> (+<0.01%)	⬆️
OS-Linux	98.68% <100.00%> (+<0.01%)	⬆️
OS-Windows	97.03% <100.00%> (+<0.01%)	⬆️
OS-macOS	97.93% <100.00%> (-0.01%)	⬇️
Py-3.10	98.12% <100.00%> (+0.01%)	⬆️
Py-3.11	98.38% <100.00%> (+<0.01%)	⬆️
Py-3.12	98.47% <100.00%> (+<0.01%)	⬆️
Py-3.13	98.45% <100.00%> (+<0.01%)	⬆️
Py-3.14	98.47% <100.00%> (+0.01%)	⬆️
Py-3.14t	97.56% <100.00%> (-0.01%)	⬇️
Py-pypy-3.11	97.40% <100.00%> (+0.01%)	⬆️
VM-macos	97.93% <100.00%> (-0.01%)	⬇️
VM-ubuntu	98.68% <100.00%> (+<0.01%)	⬆️
VM-windows	97.03% <100.00%> (+<0.01%)	⬆️
cython-coverage	38.14% <100.00%> (+0.16%)	⬆️

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[codspeed-hq]

Merging this PR will not alter performance

✅ 84 untouched benchmarks
⏩ 83 skipped benchmarks¹

---

_{Comparing waterWang:fix/content-encoding-multi-coding (5371160) with master (b084084)²}

Footnotes

1. 83 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 (b1e47ae) during the generation of this report, so b084084 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩