github/authz: validate corresponding auth provider exists

Created by: bobheadxi

This looked complicated to generalize, so just went ahead with a check directly. This is not implemented as a "serious problem" because in https://github.com/sourcegraph/customer/issues/456#issuecomment-905738522 where a misconfiguration occurred, access was not unexpectedly granted to users that should not have access.

Closes https://github.com/sourcegraph/sourcegraph/issues/24419, closes https://github.com/sourcegraph/customer/issues/456