feat: add references header to reply emails (#11719)

Co-authored-by: Muhsin Keloth <muhsinkeramam@gmail.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

spec/presenters/mail_presenter_spec.rb

@@ -46,6 +46,7 @@ RSpec.describe MailPresenter do
                                 :message_id,
                                 :multipart,
                                 :number_of_attachments,
+                                :references,
                                 :subject,
                                 :text_content,
                                 :to
@@ -100,6 +101,31 @@ RSpec.describe MailPresenter do
       end
     end
 
+    describe '#references' do
+      let(:references_mail) { create_inbound_email_from_fixture('references.eml').mail }
+      let(:mail_presenter_with_references) { described_class.new(references_mail) }
+
+      context 'when mail has references' do
+        it 'returns an array of reference IDs' do
+          expect(mail_presenter_with_references.references).to eq(['4e6e35f5a38b4_479f13bb90078178@small-app-01.mail', 'test-reference-id'])
+        end
+      end
+
+      context 'when mail has no references' do
+        it 'returns an empty array' do
+          mail_presenter = described_class.new(mail_without_in_reply_to)
+          expect(mail_presenter.references).to eq([])
+        end
+      end
+
+      context 'when references are included in serialized_data' do
+        it 'includes references in the serialized data' do
+          data = mail_presenter_with_references.serialized_data
+          expect(data[:references]).to eq(['4e6e35f5a38b4_479f13bb90078178@small-app-01.mail', 'test-reference-id'])
+        end
+      end
+    end
+
     describe 'auto_reply?' do
       let(:auto_reply_mail) { create_inbound_email_from_fixture('auto_reply.eml').mail }
       let(:auto_reply_with_auto_submitted_mail) { create_inbound_email_from_fixture('auto_reply_with_auto_submitted.eml').mail }