Fix Rails 8 lazy route loading breaking OmniAuth path matching

Rails 8 loads routes lazily, so Devise set_omniauth_path_prefix
(which runs during route generation) has not fired when OmniAuth
middleware processes the first request. The strategy falls back to
the default /auth prefix and misses /admin/auth/oidc, returning
Devise Authentication passthru 404.

Set OmniAuth.config.path_prefix eagerly in the engine initializer
after the devise.omniauth call (must be after, because that call
triggers autoload of devise/omniauth which nils the value).

Author: Fivell

lib/activeadmin/oidc/engine.rb

@@ -87,6 +87,15 @@ def controllers
                             host: nil
                           }.compact
         end
+
+        # Devise propagates omniauth_path_prefix to
+        # OmniAuth.config.path_prefix during route generation
+        # (set_omniauth_path_prefix!). On Rails 8 routes load lazily,
+        # so the OmniAuth middleware may process requests before routes
+        # are drawn and miss the prefix. Set it eagerly here.
+        # Must happen AFTER `devise.omniauth` because that call
+        # triggers autoload of devise/omniauth which nils the value.
+        ::OmniAuth.config.path_prefix = ::Devise.omniauth_path_prefix
       end
 
       initializer 'activeadmin_oidc.filter_parameters' do |app|