DEV: Serialize categories in topic lists (#23597)

At this moment, this feature is under a site setting named
lazy_load_categories.

In the future, categories will no longer be preloaded through site data.
This commit add information about categories in topic list and ensures
that data is used to display topic list items.

Parent categories are serialized too because they are necessary to
render {{category-link}}.

spec/requests/list_controller_spec.rb

@@ -223,6 +223,38 @@ RSpec.describe ListController do
         expect(response.body).not_to include(restricted_tag.name)
       end
     end
+
+    context "with lazy_load_categories" do
+      fab!(:category) { Fabricate(:category) }
+      fab!(:subcategory) { Fabricate(:category, parent_category: category) }
+
+      before { topic.update!(category: subcategory) }
+
+      it "returns categories and parent categories if true" do
+        SiteSetting.lazy_load_categories = true
+
+        get "/latest.json"
+
+        expect(response.status).to eq(200)
+        expect(response.parsed_body["topic_list"]["topics"].length).to eq(1)
+        expect(response.parsed_body["topic_list"]["topics"][0]["id"]).to eq(topic.id)
+        expect(response.parsed_body["topic_list"]["categories"].length).to eq(2)
+        expect(
+          response.parsed_body["topic_list"]["categories"].map { |c| c["id"] },
+        ).to contain_exactly(category.id, subcategory.id)
+      end
+
+      it "does not return categories if not true" do
+        SiteSetting.lazy_load_categories = false
+
+        get "/latest.json"
+
+        expect(response.status).to eq(200)
+        expect(response.parsed_body["topic_list"]["topics"].length).to eq(1)
+        expect(response.parsed_body["topic_list"]["topics"][0]["id"]).to eq(topic.id)
+        expect(response.parsed_body["topic_list"]["categories"]).to eq(nil)
+      end
+    end
   end
 
   describe "categories and X" do