Fix Cython WebSocketReader byte signedness

[bdraco]

The first_byte and second_byte are unsigned. This would occasionally cause the length to be wrong for the new WebSocketReader added in #9543 and #9554

We didn't have coverage for a longer multi-byte payload. Add coverage for this case.

[codspeed-hq]

CodSpeed Performance Report

Merging #9556 will not alter performance

_{Comparing cython_websocket_reader_unsigned (28accac) with master (ec19bfa)}

Summary

✅ 4 untouched benchmarks

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.58%. Comparing base (10f07ea) to head (28accac).
Report is 618 commits behind head on master.

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #9556   +/-   ##
=======================================
  Coverage   98.58%   98.58%           
=======================================
  Files         113      113           
  Lines       35249    35256    +7     
  Branches     4186     4186           
=======================================
+ Hits        34749    34756    +7     
  Misses        338      338           
  Partials      162      162           

Flag	Coverage Δ
CI-GHA	98.46% <100.00%> (+<0.01%)	⬆️
OS-Linux	98.13% <100.00%> (+<0.01%)	⬆️
OS-Windows	96.03% <100.00%> (-0.01%)	⬇️
OS-macOS	97.34% <100.00%> (-0.01%)	⬇️
Py-3.10.11	97.21% <100.00%> (-0.01%)	⬇️
Py-3.10.15	97.62% <100.00%> (+<0.01%)	⬆️
Py-3.11.10	97.69% <100.00%> (+<0.01%)	⬆️
Py-3.11.9	97.28% <100.00%> (-0.02%)	⬇️
Py-3.12.7	98.17% <100.00%> (+<0.01%)	⬆️
Py-3.13.0	98.16% <100.00%> (+<0.01%)	⬆️
Py-3.9.13	97.11% <100.00%> (-0.01%)	⬇️
Py-3.9.20	97.52% <100.00%> (+<0.01%)	⬆️
Py-pypy7.3.16	97.17% <100.00%> (+<0.01%)	⬆️
VM-macos	97.34% <100.00%> (-0.01%)	⬇️
VM-ubuntu	98.13% <100.00%> (+<0.01%)	⬆️
VM-windows	96.03% <100.00%> (-0.01%)	⬇️

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[bdraco]

For some reason I forget about the signedness of char but never int... but thats why I do downstream production re-testing after merge... to catch problems like this one

[patchback]

Backport to 3.11: 💔 cherry-picking failed — conflicts found

❌ Failed to cleanly apply df0c461 on top of patchback/backports/3.11/df0c46190c4d858ce2b0990320b941c22109d6b8/pr-9556

Backporting merged PR #9556 into master

1. Ensure you have a local repo clone of your fork. Unless you cloned it
   from the upstream, this would be your origin remote.
2. Make sure you have an upstream repo added as a remote too. In these
   instructions you'll refer to it by the name upstream. If you don't
   have it, here's how you can add it:

   $ git remote add upstream https://github.com/aio-libs/aiohttp.git

3. Ensure you have the latest copy of upstream and prepare a branch
   that will hold the backported code:

   $ git fetch upstream
   $ git checkout -b patchback/backports/3.11/df0c46190c4d858ce2b0990320b941c22109d6b8/pr-9556 upstream/3.11

4. Now, cherry-pick PR Fix Cython WebSocketReader byte signedness #9556 contents into that branch:

   $ git cherry-pick -x df0c46190c4d858ce2b0990320b941c22109d6b8

   If it'll yell at you with something like fatal: Commit df0c46190c4d858ce2b0990320b941c22109d6b8 is a merge but no -m option was given., add -m 1 as follows instead:

   $ git cherry-pick -m1 -x df0c46190c4d858ce2b0990320b941c22109d6b8

5. At this point, you'll probably encounter some merge conflicts. You must
   resolve them in to preserve the patch from PR Fix Cython WebSocketReader byte signedness #9556 as close to the
   original as possible.
6. Push this branch to your fork on GitHub:

   $ git push origin patchback/backports/3.11/df0c46190c4d858ce2b0990320b941c22109d6b8/pr-9556

7. Create a PR, ensure that the CI is green. If it's not — update it so that
   the tests and any other checks pass. This is it!
   Now relax and wait for the maintainers to process your pull request
   when they have some cycles to do reviews. Don't worry — they'll tell you if
   any improvements are necessary when the time comes!

🤖 @patchback
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