FEATURE: add first_post_id to TopicListItemSerializer (#31254)

Will be used in this feature:
https://github.com/discourse/discourse-topic-cards/pull/36

app/serializers/topic_list_item_serializer.rb

@@ -12,6 +12,7 @@ class TopicListItemSerializer < ListableTopicSerializer
              :op_like_count,
              :op_can_like,
              :op_liked,
+             :first_post_id,
              :pinned_globally,
              :liked_post_numbers,
              :featured_link,
@@ -70,6 +71,15 @@ class TopicListItemSerializer < ListableTopicSerializer
     ).exists?
   end
 
+  def include_first_post_id?
+    theme_modifier_helper.serialize_topic_op_likes_data
+  end
+
+  def first_post_id
+    return false if !object.first_post
+    object.first_post.id
+  end
+
   def last_poster_username
     posters.find { |poster| poster.user.id == object.last_post_user_id }.try(:user).try(:username)
   end

spec/serializers/topic_list_item_serializer_spec.rb

@@ -149,5 +149,23 @@ RSpec.describe TopicListItemSerializer do
 
       expect(json.key?(:op_liked)).to eq(false)
     end
+
+    it "serializes first_post_id" do
+      allow_any_instance_of(ThemeModifierHelper).to receive(
+        :serialize_topic_op_likes_data,
+      ).and_return(true)
+      json = TopicListItemSerializer.new(topic, scope: Guardian.new(moderator), root: false).as_json
+
+      expect(json[:first_post_id]).to eq(first_post.id)
+    end
+
+    it "does not include op_can_like when theme modifier disallows" do
+      allow_any_instance_of(ThemeModifierHelper).to receive(
+        :serialize_topic_op_likes_data,
+      ).and_return(false)
+      json = TopicListItemSerializer.new(topic, scope: Guardian.new(moderator), root: false).as_json
+
+      expect(json.key?(:first_post_id)).to eq(false)
+    end
   end
 end