Volvo's PingFederate auth API now requires a POST request for the
continueAuthentication step after OTP verification. Using GET returns
the same OTP_VERIFIED state in a loop, never progressing to COMPLETED,
which causes a KeyError on 'authorizeResponse'.
Changing auth_session.get() to auth_session.post() with an empty JSON
body causes the API to return COMPLETED with the expected authorizeResponse.
