DEV: Allow loading topic associations on /categories (#31954)

`/categories` sometimes returns accompanying topics under certain site
settings. The `CategoryList` currently allows preloading for topic
custom fields via `preloaded_topic_custom_fields`, but not for topics
themselves.

This addition is required for
https://github.com/discourse/discourse-solved/pull/342.

app/models/category_list.rb

@@ -106,7 +106,10 @@ class CategoryList
           )
     end
 
-    @all_topics = TopicQuery.remove_muted_tags(@all_topics, @guardian.user).includes(:last_poster)
+    inclusions = [:last_poster]
+    preload = DiscoursePluginRegistry.category_list_topics_preloader_associations
+    inclusions.concat(preload) if preload.present?
+    @all_topics = TopicQuery.remove_muted_tags(@all_topics, @guardian.user).includes(inclusions)
   end
 
   def find_relevant_topics

lib/discourse_plugin_registry.rb

@@ -91,6 +91,7 @@ class DiscoursePluginRegistry
 
   define_filtered_register :topic_thumbnail_sizes
   define_filtered_register :topic_preloader_associations
+  define_filtered_register :category_list_topics_preloader_associations
 
   define_filtered_register :api_parameter_routes
   define_filtered_register :api_key_scope_mappings

lib/plugin/instance.rb

@@ -1442,6 +1442,12 @@ class Plugin::Instance
     DiscoursePluginRegistry.register_topic_preloader_association(fields, self)
   end
 
+  # When loading /categories with topics, preload topic associations
+  # using register_category_list_topics_preloader_associations(:association_name)
+  def register_category_list_topics_preloader_associations(fields)
+    DiscoursePluginRegistry.register_category_list_topics_preloader_association(fields, self)
+  end
+
   private
 
   def setting_category

spec/models/category_list_spec.rb

@@ -471,4 +471,31 @@ RSpec.describe CategoryList do
       end
     end
   end
+
+  describe "with displayable topics" do
+    fab!(:category) { Fabricate(:category, num_featured_topics: 2) }
+    fab!(:topic) { Fabricate(:topic, category: category) }
+
+    it "preloads topic associations" do
+      DiscoursePluginRegistry.register_category_list_topics_preloader_association(
+        :first_post,
+        Plugin::Instance.new,
+      )
+
+      category = Fabricate(:category_with_definition)
+      Fabricate(:topic, category: category)
+
+      CategoryFeaturedTopic.feature_topics
+
+      displayable_topics =
+        CategoryList
+          .new(Guardian.new(admin), include_topics: true)
+          .categories
+          .find { |x| x.id == category.id }
+          .displayable_topics
+      expect(displayable_topics.first.association(:first_post).loaded?).to eq(true)
+
+      DiscoursePluginRegistry.reset_register!(:category_list_topics_preloader_associations)
+    end
+  end
 end