fix: preserve non-object response error bodies

[lin-hongkuan]

Summary

• Only read the error field from JSON object error responses.
• Preserve non-object JSON error bodies instead of raising a constructor-time exception.

Why

ResponseError currently assumes any decoded JSON response is a dictionary. JSON strings, arrays, or numbers are valid response bodies and should still produce a useful ResponseError.

Testing

• python -m pytest tests/test_type_serialization.py::test_response_error_uses_json_object_error_field tests/test_type_serialization.py::test_response_error_preserves_non_object_json_body -q
• git diff --check

[vaderollama]

LGTM — guards against non-dict JSON error bodies in ResponseError.__init__. Previously json.loads(error).get('error', error) would crash if the response body was a JSON string, array, or number. The isinstance(parsed_error, dict) check is the right fix. Tests cover both dict and non-dict cases.

@ParthSareen — ready to merge.