Add coverage for misc serializers (#32781)

spec/serializers/rest/admin/cohort_serializer_spec.rb

@@ -0,0 +1,19 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+RSpec.describe REST::Admin::CohortSerializer do
+  subject { serialized_record_json(record, described_class) }
+
+  let(:record) { Admin::Metrics::Retention.new('2024-01-01', '2024-01-02', 'day').cohorts.first }
+
+  describe 'serialization' do
+    it 'returns expected values' do
+      expect(subject)
+        .to include(
+          'data' => be_a(Array),
+          'period' => /2024-01-01/
+        )
+    end
+  end
+end