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.

spec/models/site_spec.rb

@@ -183,6 +183,40 @@ RSpec.describe Site do
         DiscoursePluginRegistry.clear_modifiers!
       end
     end
+
+    context "with lazy loaded categories enabled" do
+      fab!(:user)
+
+      before { SiteSetting.lazy_load_categories_groups = "#{Group::AUTO_GROUPS[:everyone]}" }
+
+      it "does not return any categories for anonymous users" do
+        site = Site.new(Guardian.new)
+
+        expect(site.categories).to eq([])
+      end
+
+      it "returns only sidebar categories and their parent categories" do
+        parent_category = Fabricate(:category)
+        category.update!(parent_category: parent_category)
+        Fabricate(:category_sidebar_section_link, linkable: category, user: user)
+
+        site = Site.new(Guardian.new(user))
+
+        expect(site.categories.map { |c| c[:id] }).to contain_exactly(
+          parent_category.id,
+          category.id,
+        )
+      end
+
+      it "returns only visible sidebar categories" do
+        Fabricate(:category_sidebar_section_link, linkable: category, user: user)
+        category.update!(read_restricted: true)
+
+        site = Site.new(Guardian.new(user))
+
+        expect(site.categories).to eq([])
+      end
+    end
   end
 
   it "omits groups user can not see" do