Add API datetime/format matcher for serialization specs, reduce factories (#33325)

spec/serializers/rest/account_serializer/field_serializer_spec.rb

@@ -6,14 +6,16 @@ RSpec.describe REST::AccountSerializer::FieldSerializer do
   subject { serialized_record_json(field, described_class) }
 
   let(:default_datetime) { DateTime.new(2024, 11, 28, 16, 20, 0) }
-  let(:user)    { Fabricate(:user) }
-  let(:account) { user.account }
+  let(:account) { Fabricate.build :account }
 
   context 'when verified_at is populated' do
     let(:field) { Account::Field.new(account, 'name' => 'Foo', 'value' => 'Bar', 'verified_at' => default_datetime) }
 
     it 'parses as RFC 3339 datetime' do
-      expect { DateTime.rfc3339(subject['verified_at']) }.to_not raise_error
+      expect(subject)
+        .to include(
+          'verified_at' => match_api_datetime_format
+        )
     end
   end
 end