DEV: Change Bookmarkable registration to DiscoursePluginRegistry (#20556)

Similar spirit to e195e6f614,
this moves the Bookmarkable registration to DiscoursePluginRegistry
so plugins which are not enabled do not register additional
bookmarkable classes.

plugins/chat/plugin.rb

@@ -280,7 +280,6 @@ after_initialize do
     Category.prepend Chat::CategoryExtension
     User.prepend Chat::UserExtension
     Jobs::UserEmail.prepend Chat::UserEmailExtension
-    Bookmark.register_bookmarkable(ChatMessageBookmarkable)
   end
 
   if Oneboxer.respond_to?(:register_local_handler)
@@ -777,6 +776,8 @@ after_initialize do
   register_user_destroyer_on_content_deletion_callback(
     Proc.new { |user| Jobs.enqueue(:delete_user_messages, user_id: user.id) },
   )
+
+  register_bookmarkable(ChatMessageBookmarkable)
 end
 
 if Rails.env == "test"

plugins/chat/spec/lib/chat_message_bookmarkable_spec.rb

@@ -11,10 +11,12 @@ describe ChatMessageBookmarkable do
   fab!(:channel) { Fabricate(:category_channel) }
 
   before do
-    Bookmark.register_bookmarkable(ChatMessageBookmarkable)
+    register_test_bookmarkable(ChatMessageBookmarkable)
     UserChatChannelMembership.create(chat_channel: channel, user: user, following: true)
   end
 
+  after { DiscoursePluginRegistry.reset! }
+
   let!(:message1) { Fabricate(:chat_message, chat_channel: channel) }
   let!(:message2) { Fabricate(:chat_message, chat_channel: channel) }
   let!(:bookmark1) do

plugins/chat/spec/models/chat_message_spec.rb

@@ -499,8 +499,6 @@ describe ChatMessage do
     end
 
     describe "bookmarks" do
-      before { Bookmark.register_bookmarkable(ChatMessageBookmarkable) }
-
       it "destroys bookmarks" do
         message_1 = Fabricate(:chat_message)
         bookmark_1 = Fabricate(:bookmark, bookmarkable: message_1)

plugins/chat/spec/plugin_helper.rb

@@ -18,9 +18,6 @@ module ChatSystemHelpers
     end
 
     Group.refresh_automatic_groups!
-
-    # this is reset after each test
-    Bookmark.register_bookmarkable(ChatMessageBookmarkable)
   end
 
   def chat_thread_chain_bootstrap(channel:, users:, messages_count: 4)