fix(multipart): ensure BodyPartReader.read() returns bytes not bytearray

[armorbreak001]

Summary

documents its return type as bytes, but internally builds the result in a bytearray and returns it directly. This means callers receive a bytearray instead of bytes, which breaks code that passes the result to json.dumps() or other APIs that don't accept bytearray.

Reproduction

From #12404:

reader = await request.multipart()
part = await reader.next()
data = await part.read(decode=True)
print(type(data))  # <class 'bytearray'>, should be <class 'bytes'>
json.dumps({"filename": data})  # TypeError!

Fix

Convert the internal bytearray to bytes before returning from read(). This is a minimal, backward-compatible change — bytes(bytearray) is a no-op copy for the common case.

Fixes #12404

[armorbreak001]

Duplicate of #12409 (already open, same issue #12404). This is the 3rd attempt on this issue.