Make Devise.mappings work with lazy loaded routes.

Starting from Rails 8.0, routes are lazy-loaded by default in test and development environments.
However, Devise's mappings are built during the routes loading phase.
To ensure it works correctly, we need to load the routes first before accessing @@mappings.

CHANGELOG.md

@@ -16,6 +16,8 @@
 
 * enhancements
   * Removed deprecations warning output for `Devise::Models::Authenticatable::BLACKLIST_FOR_SERIALIZATION` (@soartec-lab)
+  * Add Rails 8 support.
+    - Routes are lazy-loaded by default in test and development environments now so Devise loads them before `Devise.mappings` call.
 
 Please check [4-stable](https://github.com/heartcombo/devise/blob/4-stable/CHANGELOG.md)
 for previous changes.

lib/devise.rb

@@ -274,8 +274,14 @@ module Devise
   # PRIVATE CONFIGURATION
 
   # Store scopes mappings.
-  mattr_reader :mappings
   @@mappings = {}
+  def self.mappings
+    # Starting from Rails 8.0, routes are lazy-loaded by default in test and development environments.
+    # However, Devise's mappings are built during the routes loading phase.
+    # To ensure it works correctly, we need to load the routes first before accessing @@mappings.
+    Rails.application.try(:reload_routes_unless_loaded)
+    @@mappings
+  end
 
   # OmniAuth configurations.
   mattr_reader :omniauth_configs