* add override for responses api for openai compatible endpoints via model config
* emoving the definition of supported_endpoints member because even though it is used, it is not part of the interface. But defining it in the interface causes issues.
* Modifications to allow direct-endpoint tests to use responses api
* add a little more defensiveness to how people encode the configuration file
* remove dead code
* the responses api flag moved, so adjusting the code that sets it
* put back accidentally removed code
* fix type issue