[PR #13509/2d9c5c10 backport][3.14] Fix multipart base64 edge cases

[patchback]

This is a backport of PR #13509 as merged into master (2d9c5c1).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.22%. Comparing base (3d850ee) to head (1824685).
⚠️ Report is 1 commits behind head on 3.14.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             3.14   #13513      +/-   ##
==========================================
+ Coverage   96.20%   96.22%   +0.01%     
==========================================
  Files         158      159       +1     
  Lines       50914    51034     +120     
  Branches     2751     2758       +7     
==========================================
+ Hits        48983    49108     +125     
+ Misses       1752     1750       -2     
+ Partials      179      176       -3     

Flag	Coverage Δ
CI-GHA	96.22% <100.00%> (+0.01%)	⬆️
OS-Linux	96.00% <100.00%> (+0.01%)	⬆️
OS-Windows	93.73% <100.00%> (+0.02%)	⬆️
OS-macOS	95.22% <100.00%> (+0.02%)	⬆️
Py-3.10	95.42% <100.00%> (+0.02%)	⬆️
Py-3.11	95.66% <100.00%> (+0.02%)	⬆️
Py-3.12	95.73% <100.00%> (+0.01%)	⬆️
Py-3.13	95.73% <100.00%> (+0.01%)	⬆️
Py-3.14	95.82% <100.00%> (+0.02%)	⬆️
Py-3.14t	94.84% <100.00%> (+0.01%)	⬆️
Py-pypy-3.11	94.68% <100.00%> (+0.02%)	⬆️
VM-macos	95.22% <100.00%> (+0.02%)	⬆️
VM-ubuntu	96.00% <100.00%> (+0.01%)	⬆️
VM-windows	93.73% <100.00%> (+0.02%)	⬆️
cython-coverage	79.02% <32.25%> (-0.01%)	⬇️

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[greptile-apps]

Confidence Score: 5/5

The PR appears safe to merge.

No blocking failure remains.

_{Reviews (3): Last reviewed commit: "Apply suggestion from @Dreamsorcerer" | Re-trigger Greptile}

[codspeed-hq]

Merging this PR will not alter performance

✅ 72 untouched benchmarks
🆕 2 new benchmarks
⏩ 7 skipped benchmarks¹

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
🆕	test_read_base64_part[pyloop]	N/A	7.4 ms	N/A
🆕	test_read_chunk_base64_realignment[pyloop]	N/A	3.1 ms	N/A

---

_{Comparing patchback/backports/3.14/2d9c5c1062eff0034fb3ab120897a6c1b3ff7041/pr-13509 (1824685) with 3.14 (8cfbe9a)}

Footnotes

1. 7 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. ↩