FIX: Do not capture OAuth user on 2FA page (#27617)

If the `enforce_second_factor_on_external_auth` setting
is disabled and a user logs in with an OAuth method,
they don't automatically get redirected to /preferences/second-factor
on login. However, they can get there manually, and once there
they cannot leave.

This commit fixes the issue and allows them to leave
and also does some refactors to indicate to the client
what login method is used as a followup to
0e1102b3326ace878c357301e108154051d37c31

lib/auth.rb

@@ -1,6 +1,8 @@
 # frozen_string_literal: true
 
 module Auth
+  LOGIN_METHOD_OAUTH = "oauth"
+  LOGIN_METHOD_LOCAL = "local"
 end
 
 require "auth/auth_provider"