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>

app/mailers/conversation_reply_mailer.rb

@@ -4,6 +4,7 @@ class ConversationReplyMailer < ApplicationMailer
   attr_reader :large_attachments
 
   include ConversationReplyMailerHelper
+  include ReferencesHeaderBuilder
   default from: ENV.fetch('MAILER_SENDER_EMAIL', 'Chatwoot <accounts@chatwoot.com>')
   layout :choose_layout
 
@@ -160,6 +161,7 @@ class ConversationReplyMailer < ApplicationMailer
   end
 
   def conversation_reply_email_id
+    # Find the last incoming message's message_id to reply to
     content_attributes = @conversation.messages.incoming.last&.content_attributes
 
     if content_attributes && content_attributes['email'] && content_attributes['email']['message_id']
@@ -169,6 +171,10 @@ class ConversationReplyMailer < ApplicationMailer
     nil
   end
 
+  def references_header
+    build_references_header(@conversation, in_reply_to_email)
+  end
+
   def cc_bcc_emails
     content_attributes = @conversation.messages.outgoing.last&.content_attributes