Raise an error if the idp doesn't match the requested idp

[josephdecock]

If the client sends a bad/misconfigured/changed idp value, we should give some kind of error rather than redirecting to the login page forever.

Perhaps we should add a virtual method to the interaction response generator so that implementers can customize this more easily.

See DuendeSoftware/Support#1095

[brockallen]

IIRC we currently emit a log entry indicating that this is the reason for redirecting back to login.

[josephdecock]

Yes, we do have an info log:

Logger.LogInformation("Showing login: Current IdP ({currentIdp}) is not the requested IdP ({idp})", currentIdp, idp);