FIX: Preload parent categories for sidebar (#25726)

When "lazy load categories" is enabled, only the categories present in
the sidebar are preloaded. This is insufficient because the parent
categories are necessary too for the sidebar to be rendered properly.

app/models/site.rb

@@ -109,7 +109,16 @@ class Site
     if @guardian.can_lazy_load_categories?
       preloaded_category_ids = []
       if @guardian.authenticated?
-        preloaded_category_ids.concat(@guardian.user.secured_sidebar_category_ids(@guardian))
+        sidebar_category_ids = @guardian.user.secured_sidebar_category_ids(@guardian)
+        preloaded_category_ids.concat(
+          Category
+            .secured(@guardian)
+            .select(:parent_category_id)
+            .distinct
+            .where(id: sidebar_category_ids)
+            .pluck(:parent_category_id),
+        )
+        preloaded_category_ids.concat(sidebar_category_ids)
       end
     end