assert_operation_response is (incorrectly) based on the request's content type, not the response's.

https://github.com/open-api-spex/open_api_spex/blob/c1dbca17a3d64c107de18d85311a447cd27e96b2/lib/open_api_spex/test/test_assertions.ex#L157C7-L157C58

This line gets the request's content type, and uses it to fetch the response schema. I do not think this is the correct behaviour, as the response schema should only depend on the response's content type.