Fix NoMethodError on Rails 8.1 and move the ActiveAdmin 4.0 leg to it

gemfiles/activeadmin_4.0.gemfile

@@ -8,10 +8,7 @@ gemspec path: ".."
 # the Sprockets/Sassc pipeline with Propshaft + importmap + cssbundling +
 # Tailwind, so the asset gems differ entirely from the 3.5 gemfile.
 gem "activeadmin", ">= 4.0.0.beta", "< 5"
-# Blocked on Rails 8.1: railties 8.1 calls `delegate_missing_to` in
-# Rails::Initializable::Collection without requiring the ActiveSupport
-# core ext, so this gem's `require "rails/railtie"` raises NoMethodError.
-gem "rails",       "~> 8.0.0"
+gem "rails",       "~> 8.1.0"
 
 # openid_connect 2.x line (faraday 2.x).
 gem "omniauth_openid_connect", "~> 0.8.0"
@@ -20,7 +17,3 @@ gem "propshaft"
 gem "importmap-rails"
 gem "cssbundling-rails"
 gem "tailwindcss-rails", "~> 4.0"
-
-# See the note in activeadmin_3.5.gemfile: json 3.0 removed the
-# `quirks_mode` keyword ActiveSupport::JSON still passes.
-gem "json", "< 3"

lib/activeadmin-oidc.rb

@@ -18,6 +18,17 @@
 # at file load, so pull in the minimal Railtie base class first — this
 # keeps the require safe even in spec_helper contexts where the full
 # Rails stack hasn't been initialized yet.
+#
+# The delegation core ext has to come first. railties 8.1's
+# `rails/initializable.rb` calls `delegate_missing_to` in the body of
+# `Rails::Initializable::Collection` but only requires "tsort", and
+# `rails/railtie.rb` requires `rails/initializable` before any
+# ActiveSupport core ext. So on Rails 8.1 a bare `require
+# "rails/railtie"` raises `NoMethodError: undefined method
+# 'delegate_missing_to'`. Requiring the one core ext it needs keeps the
+# intent above intact — we still load only the minimal Railtie base
+# class, not all of Rails.
+require "active_support/core_ext/module/delegation"
 require "rails/railtie"
 require "omniauth/rails_csrf_protection"