DEV: Set topic list filter name in serializer for children (#29291)

spec/serializers/topic_list_serializer_spec.rb

@@ -14,4 +14,13 @@ RSpec.describe TopicListSerializer do
     expect(serialized[:primary_groups]).to eq([])
     expect(serialized[:topic_list][:topics].first[:id]).to eq(topic.id)
   end
+
+  it "adds filter name to the options hash so childrens can access it" do
+    filter = :hot
+    topic_list = TopicList.new(filter, user, [topic])
+
+    serializer = described_class.new(topic_list, scope: Guardian.new(user))
+
+    expect(serializer.options[:filter]).to eq(filter)
+  end
 end