fix(client): avoid cross-loop eager body writers

[daleselaji-dev]

What do these changes do?

Guard aiohttp's Python 3.12+ eager request-body writer optimization so it is used only when the request is sent from the session's event loop. Cross-loop use now schedules the writer on the session loop instead of eagerly stepping a coroutine from the caller's thread.

Are there changes in behavior for the user?

Same-loop requests keep the existing eager-write optimization. Cross-loop misuse no longer lets a foreign thread execute a task against the session loop, avoiding the event-loop corruption described in #13346.

Related issue number

Fixes #13346

Testing

• Added a regression test asserting that the eager task constructor is not used when the caller loop differs.
• python -m compileall -q aiohttp/client_reqrep.py tests/test_client_request.py passed.
• The focused pytest run was blocked locally because the Windows checkout cannot build aiohttp's generated C extensions; GitHub CI will provide the supported build/test environment.
• Added CHANGES/13346.bugfix.rst.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.99%. Comparing base (288cf46) to head (47ca267).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #13359   +/-   ##
=======================================
  Coverage   98.99%   98.99%           
=======================================
  Files         132      132           
  Lines       49156    49164    +8     
  Branches     2560     2560           
=======================================
+ Hits        48661    48669    +8     
  Misses        371      371           
  Partials      124      124           

Flag	Coverage Δ
Autobahn	22.10% <11.11%> (-0.01%)	⬇️
CI-GHA	98.90% <100.00%> (+<0.01%)	⬆️
OS-Linux	98.67% <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% <88.88%> (+<0.01%)	⬆️
Py-3.11	98.37% <88.88%> (-0.01%)	⬇️
Py-3.12	98.47% <100.00%> (+<0.01%)	⬆️
Py-3.13	98.45% <100.00%> (+<0.01%)	⬆️
Py-3.14	98.46% <100.00%> (-0.01%)	⬇️
Py-3.14t	97.56% <100.00%> (-0.01%)	⬇️
Py-pypy-3.11	97.40% <88.88%> (-0.02%)	⬇️
VM-macos	97.93% <100.00%> (+<0.01%)	⬆️
VM-ubuntu	98.67% <100.00%> (+<0.01%)	⬆️
VM-windows	97.03% <100.00%> (+<0.01%)	⬆️
cython-coverage	38.06% <0.00%> (-0.01%)	⬇️

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[codspeed-hq]

Merging this PR will not alter performance

✅ 84 untouched benchmarks
⏩ 83 skipped benchmarks¹

---

_{Comparing daleselaji-dev:codex/fix-cross-loop-eager-write (47ca267) with master (288cf46)}

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. ↩