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.

app/models/concerns/message_filter_helpers.rb

@@ -14,7 +14,7 @@ module MessageFilterHelpers
   end
 
   def notifiable?
-    incoming? || outgoing?
+    (incoming? || outgoing?) && !private?
   end
 
   def conversation_transcriptable?