Modifications to allow direct-endpoint tests to use responses api (#1047)

* 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