Fix BodyPartReader.read() returning bytearray instead of bytes

[terminalchai]

Problem

BodyPartReader.read() accumulates data into a bytearray internally but returns it directly without converting to bytes, violating the documented return type.

This affects both code paths in read():

• decode=False: returns the raw bytearray accumulation
• decode=True: returns the decoded bytearray accumulation

The result is a silent type error downstream. For example, trying to JSON-serialize the result fails:

data = await part.read()
json.dumps({'content': data})
# TypeError: Object of type bytearray is not JSON serializable

Fixes #12404.

Fix

Wrap both return values in bytes():

return bytes(result)

One-line change on each code path. Two regression tests added to tests/test_multipart.py covering both decode=False and decode=True.

[codspeed-hq]

Merging this PR will not alter performance

✅ 67 untouched benchmarks
⏩ 4 skipped benchmarks¹

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_{Comparing terminalchai:fix/multipart-read-returns-bytes (3dda5cb) with master (7d534ab)}

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. ↩