Handle temporary model issues

[kieferrm]

GH will change 503 json return

{
  "error" : {
     "message" : "some message",
     "code" : "some_error_code"
  }
}

to support model_overloaded and model_degraded. We'll need to handle these new error code appropriately.

We're also discussing that the list of models we fetch should support the same annotations that we then should render in the model picker. If so, we should re-fetch the model list when we receive a 503 with aforementioned error codes and update the model picker.

[natefinch]

PR is here, with a description of the two errors:

https://github.com/github/copilot-api/pull/18303/

[natefinch]

The capi code has now shipped :)