[3.11] Fix WebSocket reader flow control size calculation for multi-byte data

[bdraco]

The decoded size was passed to the FlowControlDataQueue instead of the bytes size which meant multi-byte characters would be under counted.

Note that this code is completely different on 3.11/3.10 than master so this PR is directly to 3.11

[codspeed-hq]

CodSpeed Performance Report

Merging #9686 will not alter performance

_{Comparing multi_byte_flow_control_fix_311 (3eb8a69) with 3.11 (685768a)}

Summary

✅ 14 untouched benchmarks

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.92%. Comparing base (685768a) to head (3eb8a69).
Report is 230 commits behind head on 3.11.

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             3.11    #9686   +/-   ##
=======================================
  Coverage   97.92%   97.92%           
=======================================
  Files         117      117           
  Lines       35845    35844    -1     
  Branches     4295     4294    -1     
=======================================
  Hits        35101    35101           
+ Misses        558      557    -1     
  Partials      186      186           

Flag	Coverage Δ
CI-GHA	97.81% <100.00%> (+<0.01%)	⬆️
OS-Linux	97.50% <100.00%> (+<0.01%)	⬆️
OS-Windows	94.42% <100.00%> (+<0.01%)	⬆️
OS-macOS	96.72% <100.00%> (+<0.01%)	⬆️
Py-3.10.11	96.57% <100.00%> (+<0.01%)	⬆️
Py-3.10.15	97.00% <100.00%> (+<0.01%)	⬆️
Py-3.11.10	97.06% <100.00%> (+<0.01%)	⬆️
Py-3.11.9	96.65% <100.00%> (+<0.01%)	⬆️
Py-3.12.7	97.54% <100.00%> (+<0.01%)	⬆️
Py-3.13.0	97.52% <100.00%> (+<0.01%)	⬆️
Py-3.9.13	96.50% <100.00%> (+<0.01%)	⬆️
Py-3.9.20	96.93% <100.00%> (+<0.01%)	⬆️
Py-pypy7.3.16	96.53% <100.00%> (-0.01%)	⬇️
VM-macos	96.72% <100.00%> (+<0.01%)	⬆️
VM-ubuntu	97.50% <100.00%> (+<0.01%)	⬆️
VM-windows	94.42% <100.00%> (+<0.01%)	⬆️

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[bdraco]

https://www.merriam-webster.com/dictionary/undercount

Hmm.. guess its not in the spelling words

[patchback]

Backport to 3.10: 💔 cherry-picking failed — conflicts found

❌ Failed to cleanly apply ea1c084 on top of patchback/backports/3.10/ea1c084d9f49e91feb2e92fea307260ad3f47376/pr-9686

Backporting merged PR #9686 into 3.11

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/3.10/ea1c084d9f49e91feb2e92fea307260ad3f47376/pr-9686 upstream/3.10

4. Now, cherry-pick PR [3.11] Fix WebSocket reader flow control size calculation for multi-byte data #9686 contents into that branch:

   $ git cherry-pick -x ea1c084d9f49e91feb2e92fea307260ad3f47376

   If it'll yell at you with something like fatal: Commit ea1c084d9f49e91feb2e92fea307260ad3f47376 is a merge but no -m option was given., add -m 1 as follows instead:

   $ git cherry-pick -m1 -x ea1c084d9f49e91feb2e92fea307260ad3f47376

5. At this point, you'll probably encounter some merge conflicts. You must
   resolve them in to preserve the patch from PR [3.11] Fix WebSocket reader flow control size calculation for multi-byte data #9686 as close to the
   original as possible.
6. Push this branch to your fork on GitHub:

   $ git push origin patchback/backports/3.10/ea1c084d9f49e91feb2e92fea307260ad3f47376/pr-9686

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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