ensure Signed In
Checks if the user is signed in and invokes the callback if needed.
This method performs the following:
If no callback is set, returns immediately
Checks if the user is signed in using SudoUserClient.isSignedIn
If not signed in, invokes the callback's SudoPlatformSignInCallback.signIn method
Throws
Any error thrown by the callback's signIn() method, or errors from checking sign-in status.