Ensures that the last read body part is read completely.

async def _maybe_release_last_part(self) -> None:
        """Ensures that the last read body part is read completely."""
        if self._last_part is not None:
            if not self._last_part.at_eof():
                await self._last_part.release()
            self._unread.extend(self._last_part._unread)
            self._last_part = None