Dear Kamala,
The OOTB /userinfo only returns a hardcoded user "Darth Vader", only the 'sub', 'preferred_username' and 'user_role' are real. That's because otk will not know which IDP (user store) you're going to use, therefore it cannot return the real user attributes.
You would need to modify OTK User Attribute Look Up Extension policy to retrieve the real user attributes, rather than "invoke an internal rest service", you would just create ldap IDP to connect to your ldap user store, and use Query LDAP assertion to query the user information.
Regards,
Mark