Add default response is not breaking change

この issue には説明がありません。

The issue body names no files, tests, or entry points, so first inspect the openapi-diff command and the code that classifies response changes. Confirm the expected treatment of a default response with the maintainers, then add or update coverage showing that this case is not reported as a breaking change.

索引モデルが issue の本文から書いたものです。