Add coverage for REST::AnnualReportEventSerializer and two ActivityPub::* serializers (#33355)

Co-authored-by: Claire <claire.github-309c@sitedethib.com>

spec/serializers/rest/annual_report_event_serializer_spec.rb

@@ -0,0 +1,18 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+RSpec.describe REST::AnnualReportEventSerializer do
+  subject { serialized_record_json(record, described_class) }
+
+  describe 'serializing an object' do
+    let(:record) { Fabricate.build :generated_annual_report, year: 2024 }
+
+    it 'returns expected attributes' do
+      expect(subject.deep_symbolize_keys)
+        .to include(
+          year: '2024'
+        )
+    end
+  end
+end