FIX: Show deleted bookmark reminders in user bookmarks menu (#25905)

When we send a bookmark reminder, there is an option to delete
the underlying bookmark. The Notification record stays around.
However, if you want to filter your notifications user menu
to only bookmark-based notifications, we were not showing unread
bookmark notifications for deleted bookmarks.

This commit fixes the issue _going forward_ by adding the
bookmarkable_id and bookmarkable_type to the Notification data,
so we can look up the underlying Post/Topic/Chat::Message
for a deleted bookmark and check user access in this way. Then,
it doesn't matter if the bookmark was deleted.

spec/requests/users_controller_spec.rb

@@ -7087,6 +7087,37 @@ RSpec.describe UsersController do
         expect(notifications.first["data"]["bookmark_id"]).to eq(bookmark_with_reminder.id)
       end
 
+      it "shows unread notifications even if the bookmark has been deleted if they have bookmarkable data" do
+        bookmark_with_reminder.destroy!
+
+        get "/u/#{user.username}/user-menu-bookmarks"
+        expect(response.status).to eq(200)
+
+        notifications = response.parsed_body["notifications"]
+        expect(notifications.size).to eq(1)
+        expect(notifications.first["data"]["bookmark_id"]).to eq(bookmark_with_reminder.id)
+      end
+
+      it "does not show unread notifications if the bookmark has been deleted if they only have the bookmark_id data" do
+        notif =
+          Notification.find_by(
+            topic: bookmark_with_reminder.bookmarkable.topic,
+            post_number: bookmark_with_reminder.bookmarkable.post_number,
+          )
+        new_data = notif.data_hash
+        new_data.delete(:bookmarkable_type)
+        new_data.delete(:bookmarkable_id)
+        notif.update!(data: JSON.dump(new_data))
+
+        bookmark_with_reminder.destroy!
+
+        get "/u/#{user.username}/user-menu-bookmarks"
+        expect(response.status).to eq(200)
+
+        notifications = response.parsed_body["notifications"]
+        expect(notifications.size).to eq(0)
+      end
+
       context "with `show_user_menu_avatars` setting enabled" do
         before { SiteSetting.show_user_menu_avatars = true }