FIX: Chat NotificationLevels extension breaking in prod (#21484)

When setting DISCOURSE_ZEITWERK_EAGER_LOAD=1 to enable
eager loading the previous solution to adding chat_levels
to the core NotificationLevels would break with a module
loading error (c.f. cc2570fce3)

We don't actually _need_ to extend the core class, we can just
make our own for chat, let's do this instead.

plugins/chat/app/models/chat/user_chat_thread_membership.rb

@@ -8,7 +8,7 @@ module Chat
     belongs_to :last_read_message, class_name: "Chat::Message", optional: true
     belongs_to :thread, class_name: "Chat::Thread", foreign_key: :thread_id
 
-    enum :notification_level, NotificationLevels.chat_levels
+    enum :notification_level, Chat::NotificationLevels.all
   end
 end
 

plugins/chat/lib/chat/notification_levels.rb

@@ -0,0 +1,9 @@
+# frozen_string_literal: true
+
+module Chat
+  class NotificationLevels
+    def self.all
+      @all_levels ||= Enum.new(muted: 0, normal: 1, tracking: 2, watching: 3)
+    end
+  end
+end

plugins/chat/lib/chat/notification_levels_extension.rb

@@ -1,13 +0,0 @@
-# frozen_string_literal: true
-
-module Chat
-  module NotificationLevelsExtension
-    extend ActiveSupport::Concern
-
-    class_methods do
-      def chat_levels
-        @chat_levels ||= Enum.new(muted: 0, normal: 1, tracking: 2, watching: 3)
-      end
-    end
-  end
-end

plugins/chat/plugin.rb

@@ -63,7 +63,6 @@ after_initialize do
     User.prepend Chat::UserExtension
     Jobs::UserEmail.prepend Chat::UserEmailExtension
     Plugin::Instance.prepend Chat::PluginInstanceExtension
-    NotificationLevels.prepend Chat::NotificationLevelsExtension
   end
 
   if Oneboxer.respond_to?(:register_local_handler)