fix: Skip notifications for private notes (#13617)

When agents or integrations create private notes on a conversation,
every note was sending a notification to the assigned agent and all
conversation participants. The fix ensures that private notes no longer
trigger new message notifications. If someone explicitly mentions a
teammate in a private note, that person will still get notified as
expected.

spec/services/messages/new_message_notification_service_spec.rb

@@ -2,11 +2,17 @@ require 'rails_helper'
 
 describe Messages::NewMessageNotificationService do
   context 'when message is not notifiable' do
-    it 'will not create any notifications' do
+    it 'will not create any notifications for activity messages' do
       message = build(:message, message_type: :activity)
       expect(NotificationBuilder).not_to receive(:new)
       described_class.new(message: message).perform
     end
+
+    it 'will not create any notifications for private messages' do
+      message = build(:message, message_type: :outgoing, private: true)
+      expect(NotificationBuilder).not_to receive(:new)
+      described_class.new(message: message).perform
+    end
   end
 
   context 'when message is notifiable' do