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/mailers/conversation_reply_mailer_spec.rb

@@ -137,6 +137,99 @@ RSpec.describe ConversationReplyMailer do
       end
     end
 
+    context 'with references header' do
+      let(:conversation) { create(:conversation, assignee: agent, inbox: email_channel.inbox, account: account).reload }
+      let(:message) { create(:message, conversation: conversation, account: account, message_type: 'outgoing', content: 'Outgoing Message 2') }
+      let(:mail) { described_class.email_reply(message).deliver_now }
+
+      context 'when starting a new conversation' do
+        let(:first_outgoing_message) do
+          create(:message,
+                 conversation: conversation,
+                 account: account,
+                 message_type: 'outgoing',
+                 content: 'First outgoing message')
+        end
+        let(:mail) { described_class.email_reply(first_outgoing_message).deliver_now }
+
+        it 'has only the conversation reference' do
+          # When starting a conversation, references will have the default conversation ID
+          # Extract domain from the actual references header to handle dynamic domain selection
+          actual_domain = mail.references.split('@').last
+          expected_reference = "account/#{account.id}/conversation/#{conversation.uuid}@#{actual_domain}"
+          expect(mail.references).to eq(expected_reference)
+        end
+      end
+
+      context 'when replying to a message with no references' do
+        let(:incoming_message) do
+          create(:message,
+                 conversation: conversation,
+                 account: account,
+                 message_type: 'incoming',
+                 source_id: '<incoming-123@example.com>',
+                 content: 'Incoming message',
+                 content_attributes: {
+                   'email' => {
+                     'message_id' => 'incoming-123@example.com'
+                   }
+                 })
+        end
+        let(:reply_message) do
+          create(:message,
+                 conversation: conversation,
+                 account: account,
+                 message_type: 'outgoing',
+                 content: 'Reply to incoming')
+        end
+        let(:mail) { described_class.email_reply(reply_message).deliver_now }
+
+        before do
+          incoming_message
+        end
+
+        it 'includes only the in_reply_to id in references' do
+          # References should only have the incoming message ID when no prior references exist
+          expect(mail.references).to eq('incoming-123@example.com')
+        end
+      end
+
+      context 'when replying to a message that has references' do
+        let(:incoming_message_with_refs) do
+          create(:message,
+                 conversation: conversation,
+                 account: account,
+                 message_type: 'incoming',
+                 source_id: '<incoming-456@example.com>',
+                 content: 'Incoming with references',
+                 content_attributes: {
+                   'email' => {
+                     'message_id' => 'incoming-456@example.com',
+                     'references' => ['<ref-1@example.com>', '<ref-2@example.com>']
+                   }
+                 })
+        end
+        let(:reply_message) do
+          create(:message,
+                 conversation: conversation,
+                 account: account,
+                 message_type: 'outgoing',
+                 content: 'Reply to message with refs')
+        end
+        let(:mail) { described_class.email_reply(reply_message).deliver_now }
+
+        before do
+          incoming_message_with_refs
+        end
+
+        it 'includes existing references plus the in_reply_to id' do
+          # Rails returns references as an array when multiple values are present
+          expected_references = ['ref-1@example.com', 'ref-2@example.com', 'incoming-456@example.com']
+          expect(mail.references).to eq(expected_references)
+        end
+      end
+    end
+
     context 'with email reply' do
       let(:conversation) { create(:conversation, assignee: agent, inbox: email_channel.inbox, account: account).reload }
       let(:message) { create(:message, conversation: conversation, account: account, message_type: 'outgoing', content: 'Outgoing Message 2') }