DEV: Better categories pagination (#26421)

Pagination is enabled only when "lazy load categories" is enabled. For
those cases when it is not, the first page should return all the
results.

spec/requests/categories_controller_spec.rb

@@ -397,6 +397,35 @@ RSpec.describe CategoriesController do
         response.parsed_body["category_list"]["categories"].map { |x| x["id"] },
       ).not_to include(uncategorized.id)
     end
+
+    describe "with page" do
+      before { sign_in(admin) }
+
+      let!(:category2) { Fabricate(:category, user: admin) }
+      let!(:category3) { Fabricate(:category, user: admin) }
+
+      it "paginates results wihen lazy_load_categories is enabled" do
+        SiteSetting.lazy_load_categories_groups = "#{Group::AUTO_GROUPS[:everyone]}"
+
+        stub_const(CategoryList, "CATEGORIES_PER_PAGE", 2) { get "/categories.json?page=1" }
+        expect(response.status).to eq(200)
+        expect(response.parsed_body["category_list"]["categories"].count).to eq(2)
+
+        stub_const(CategoryList, "CATEGORIES_PER_PAGE", 2) { get "/categories.json?page=2" }
+        expect(response.status).to eq(200)
+        expect(response.parsed_body["category_list"]["categories"].count).to eq(2)
+      end
+
+      it "does not paginate results when lazy_load_categories is disabled" do
+        stub_const(CategoryList, "CATEGORIES_PER_PAGE", 2) { get "/categories.json?page=1" }
+        expect(response.status).to eq(200)
+        expect(response.parsed_body["category_list"]["categories"].count).to eq(4)
+
+        stub_const(CategoryList, "CATEGORIES_PER_PAGE", 2) { get "/categories.json?page=2" }
+        expect(response.status).to eq(200)
+        expect(response.parsed_body["category_list"]["categories"].count).to eq(0)
+      end
+    end
   end
 
   describe "extensibility event" do