fix: ensure BodyPartReader.read() returns bytes instead of bytearray

The read() method's return type annotation promises bytes, but both
the decoded and raw paths returned the internal bytearray buffer.
This caused TypeError with json.dumps and other serializers that
don't accept bytearray.

Convert both return values to bytes() to match the documented API.

Author: armorbreak001

aiohttp/multipart.py

@@ -322,8 +322,8 @@ async def read(self, *, decode: bool = False) -> bytes:
                 decoded_data.extend(d)
                 if len(decoded_data) > self._client_max_size:
                     raise self._max_size_error_cls(self._client_max_size)
-            return decoded_data
-        return data
+            return bytes(decoded_data)
+        return bytes(data)
 
     async def read_chunk(self, size: int = chunk_size) -> bytes:
         """Reads body part content chunk of the specified size.