chore: Add a condition to handle bounced email (#11873)

Add bounced emails to the conversation thread.
Fix Gmail bounce detection by checking the X-Failed-Recipients header.

Currently, bounced emails are rejected as auto-replies, which causes
support agents to miss important delivery failure context. This PR
ensures bounced messages are correctly added to the thread, preserving
visibility for the support team.

spec/mailboxes/imap/imap_mailbox_spec.rb

@@ -115,6 +115,14 @@ RSpec.describe Imap::ImapMailbox do
       end
     end
 
+    context 'when the email is bounced' do
+      let!(:bounced_mail) { create_inbound_email_from_fixture('bounced_gmail.eml') }
+
+      it 'processes the bounced email' do
+        expect { class_instance.process(bounced_mail.mail, channel) }.to change(Message, :count)
+      end
+    end
+
     context 'when a reply for existing email conversation' do
       let(:prev_conversation) { create(:conversation, account: account, inbox: channel.inbox, assignee: agent) }
       let(:reply_mail) do