Support OpenAPI specifications which only have a default response #60
Description
The Google Generative AI OpenAPI specification only has a default response for every operation (I don't know why, it doesn't make a whole lot of sense). At the moment, the codegen will silently ignore this and instead return handlers that always return an unexpected status.
I attached the Google Generative AI OpenAPI spec to this issue as a reference.
I've started some work to try to support the default response in #59, but I'm not sure how to surface this properly in the generated client interface.