Add coverage for misc serializers (#32781)

spec/serializers/activitypub/add_serializer_spec.rb

@@ -0,0 +1,27 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+RSpec.describe ActivityPub::AddSerializer do
+  describe '.serializer_for' do
+    subject { described_class.serializer_for(model, {}) }
+
+    context 'with a Status model' do
+      let(:model) { Status.new }
+
+      it { is_expected.to eq(described_class::UriSerializer) }
+    end
+
+    context 'with a FeaturedTag model' do
+      let(:model) { FeaturedTag.new }
+
+      it { is_expected.to eq(ActivityPub::HashtagSerializer) }
+    end
+
+    context 'with an Array' do
+      let(:model) { [] }
+
+      it { is_expected.to eq(ActiveModel::Serializer::CollectionSerializer) }
+    end
+  end
+end