Fix BodyPartReader.read() returning bytearray instead of bytes

[mitre88]

BodyPartReader.read() was returning the internal bytearray buffer directly instead of bytes, violating the annotated return type of bytes.

This caused TypeError: Object of type bytearray is not JSON serializable when downstream code (like json.dump) tried to serialize the value.

The fix wraps the return values with bytes() to match the annotated return type.

Fixes #12404

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.92%. Comparing base (0471d65) to head (ddf2785).
⚠️ Report is 2 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #12466   +/-   ##
=======================================
  Coverage   98.92%   98.92%           
=======================================
  Files         134      134           
  Lines       46750    46750           
  Branches     2429     2429           
=======================================
  Hits        46248    46248           
  Misses        373      373           
  Partials      129      129           

Flag	Coverage Δ
CI-GHA	98.98% <100.00%> (ø)
OS-Linux	98.72% <100.00%> (-0.01%)	⬇️
OS-Windows	96.98% <100.00%> (ø)
OS-macOS	97.89% <100.00%> (+<0.01%)	⬆️
Py-3.10.11	97.39% <100.00%> (ø)
Py-3.10.20	97.86% <100.00%> (+<0.01%)	⬆️
Py-3.11.15	98.11% <100.00%> (ø)
Py-3.11.9	97.64% <100.00%> (-0.01%)	⬇️
Py-3.12.10	97.74% <100.00%> (+<0.01%)	⬆️
Py-3.12.13	98.20% <100.00%> (+<0.01%)	⬆️
Py-3.13.13	98.44% <100.00%> (-0.01%)	⬇️
Py-3.14.4	98.51% <100.00%> (ø)
Py-3.14.4t	97.51% <100.00%> (ø)
Py-pypy3.11.15-7.3.21	97.35% <100.00%> (ø)
VM-macos	97.89% <100.00%> (+<0.01%)	⬆️
VM-ubuntu	98.72% <100.00%> (-0.01%)	⬇️
VM-windows	96.98% <100.00%> (ø)
cython-coverage	38.08% <100.00%> (ø)

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[codspeed-hq]

Merging this PR will not alter performance

✅ 67 untouched benchmarks
⏩ 4 skipped benchmarks¹

---

_{Comparing mitre88:fix/bodyreader-bytearray-leak (ddf2785) with master (0471d65)²}

Footnotes

1. 4 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. ↩

2. No successful run was found on master (0f0b0e5) during the generation of this report, so 0471d65 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩