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

The read() method accumulated data in a bytearray but returned it
directly, violating the documented return type annotation (bytes).
This caused TypeError when passing the result to json.dumps or
other APIs that expect bytes rather than bytearray.

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.