Fix BodyPartReader.read() returning bytearray instead of bytes

[jesustorres-code]

Fixes #12404

Problem

BodyPartReader.read() accumulates data in a bytearray internally and returns it directly, violating the -> bytes return type annotation. With decode=True the decoded accumulation buffer also returns as bytearray.

This causes downstream failures like TypeError: Object of type bytearray is not JSON serializable when the result is passed to json.dumps, and silently breaks code that checks isinstance(result, bytes).

Fix

Convert the internal bytearray to bytes at the return point in both code paths:

# before
return decoded_data  # bytearray
return data          # bytearray

# after
return bytes(decoded_data)
return bytes(data)

The bytearray is still used internally for efficient .extend() accumulation; only the final return value changes.

Tests

• Added assert type(result) is bytes to the existing test_read test
• Added test_read_returns_bytes_not_bytearray_with_decode covering the decode=True path with Content-Encoding: identity

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.56%. Comparing base (a0a03ca) to head (0fa69e9).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #12671      +/-   ##
==========================================
- Coverage   98.95%   98.56%   -0.39%     
==========================================
  Files         131      131              
  Lines       46688    46696       +8     
  Branches     2421     2421              
==========================================
- Hits        46200    46026     -174     
- Misses        366      526     +160     
- Partials      122      144      +22     

Flag	Coverage Δ
Autobahn	22.41% <10.00%> (-0.01%)	⬇️
CI-GHA	98.01% <100.00%> (-0.91%)	⬇️
OS-Linux	97.69% <100.00%> (-0.99%)	⬇️
OS-Windows	?
OS-macOS	97.93% <100.00%> (-0.01%)	⬇️
Py-3.10	97.20% <100.00%> (-0.96%)	⬇️
Py-3.11	97.44% <100.00%> (-0.97%)	⬇️
Py-3.12	97.53% <100.00%> (-0.97%)	⬇️
Py-3.13	97.61% <100.00%> (-0.87%)	⬇️
Py-3.14	97.63% <100.00%> (-0.87%)	⬇️
Py-3.14t	?
Py-pypy-3.11	?
VM-macos	97.93% <100.00%> (-0.01%)	⬇️
VM-ubuntu	97.69% <100.00%> (-0.99%)	⬇️
VM-windows	?
cython-coverage	37.93% <20.00%> (-0.01%)	⬇️

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[codspeed-hq]

Merging this PR will not alter performance

✅ 72 untouched benchmarks
⏩ 72 skipped benchmarks¹

---

_{Comparing jesustorres-code:fix/body-part-reader-returns-bytes (0fa69e9) with master (a0a03ca)}

Footnotes

1. 72 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. ↩