[PR #10037/2e369db backport][3.12] Refactor requests and responses to use classvar defaults to avoid multiple __init__s

[bdraco]

(cherry picked from commit 2e369db)

[codspeed-hq]

CodSpeed Performance Report

Merging #10054 will improve performances by 15.5%

_{Comparing patchback/backports/3.12/2e369db2d9abcbef41031592b4f044adf5f89f59/pr-10037 (8e5482e) with 3.12 (c9eb8e7)}

Summary

⚡ 1 improvements
✅ 43 untouched benchmarks

Benchmarks breakdown

	Benchmark	3.12	patchback/backports/3.12/2e369db2d9abcbef41031592b4f044adf5f89f59/pr-10037	Change
⚡	test_simple_web_stream_response	318.7 µs	276 µs	+15.5%

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.03%. Comparing base (c9eb8e7) to head (8e5482e).
Report is 1 commits behind head on 3.12.

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             3.12   #10054      +/-   ##
==========================================
+ Coverage   96.38%   98.03%   +1.65%     
==========================================
  Files         120      122       +2     
  Lines       36865    36931      +66     
  Branches     4424     4448      +24     
==========================================
+ Hits        35533    36207     +674     
+ Misses       1073      543     -530     
+ Partials      259      181      -78     

Flag	Coverage Δ
CI-GHA	97.93% <100.00%> (+1.54%)	⬆️
OS-Linux	97.63% <100.00%> (?)
OS-Windows	94.53% <100.00%> (?)
OS-macOS	96.74% <100.00%> (+0.35%)	⬆️
Py-3.10.11	96.60% <100.00%> (?)
Py-3.10.15	97.19% <100.00%> (?)
Py-3.11.10	97.25% <100.00%> (?)
Py-3.11.9	96.67% <100.00%> (+0.28%)	⬆️
Py-3.12.7	97.70% <100.00%> (?)
Py-3.13.0	97.68% <100.00%> (?)
Py-3.9.13	96.53% <100.00%> (?)
Py-3.9.20	97.10% <100.00%> (?)
Py-pypy7.3.16	96.67% <100.00%> (?)
VM-macos	96.74% <100.00%> (+0.35%)	⬆️
VM-ubuntu	97.63% <100.00%> (?)
VM-windows	94.53% <100.00%> (?)

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