[native] validate auth result on identity register

Summary:
just adding the same assertWithValidator call that we have in logInPasswordUser and getKeyserverKeys

Depends on D10809

Test Plan: registered a new user from native and got back the auth result, which means the validation succeeded

Reviewers: tomek

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D10815