[PR #8657/6c6ecfaf backport][3.11] Fix multipart reading with split boundary

[patchback]

This is a backport of PR #8657 as merged into master (6c6ecfa).

Fixes #8643.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.71%. Comparing base (dce075a) to head (742d323).
Report is 524 commits behind head on 3.11.

Additional details and impacted files

@@           Coverage Diff           @@
##             3.11    #8659   +/-   ##
=======================================
  Coverage   97.71%   97.71%           
=======================================
  Files         108      108           
  Lines       33922    33970   +48     
  Branches     4036     4044    +8     
=======================================
+ Hits        33146    33194   +48     
  Misses        579      579           
  Partials      197      197           

Flag	Coverage Δ
CI-GHA	97.61% <100.00%> (+<0.01%)	⬆️
OS-Linux	97.29% <100.00%> (+<0.01%)	⬆️
OS-Windows	94.81% <100.00%> (+<0.01%)	⬆️
OS-macOS	96.96% <100.00%> (+<0.01%)	⬆️
Py-3.10.11	97.06% <100.00%> (+<0.01%)	⬆️
Py-3.10.14	97.01% <100.00%> (+<0.01%)	⬆️
Py-3.11.9	97.25% <100.00%> (+<0.01%)	⬆️
Py-3.12.4	97.35% <100.00%> (+<0.01%)	⬆️
Py-3.8.10	94.43% <21.56%> (-0.12%)	⬇️
Py-3.8.18	96.79% <21.56%> (-0.12%)	⬇️
Py-3.9.13	96.94% <21.56%> (-0.12%)	⬇️
Py-3.9.19	96.90% <21.56%> (-0.12%)	⬇️
Py-pypy7.3.16	96.49% <21.56%> (-0.12%)	⬇️
VM-macos	96.96% <100.00%> (+<0.01%)	⬆️
VM-ubuntu	97.29% <100.00%> (+<0.01%)	⬆️
VM-windows	94.81% <100.00%> (+<0.01%)	⬆️

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