fix: skip self-mention notification in private notes (#14318)

When an agent mentions themselves in a private note, they no longer
receive a redundant notification for their own mention.

Closes: #4096

# Pull Request Template

## Description

Agents who mention themselves in a private note no longer receive a
conversation_mention notification. Previously, the mention service would
generate a notification for every mentioned user without checking
whether the sender and the
mentioned user were the same person.

app/services/messages/mention_service.rb

@@ -50,6 +50,8 @@ class Messages::MentionService
 
   def generate_notifications_for_mentions(validated_mentioned_ids)
     validated_mentioned_ids.each do |user_id|
+      next if self_mention?(user_id)
+
       NotificationBuilder.new(
         notification_type: 'conversation_mention',
         user: User.find(user_id),
@@ -60,6 +62,10 @@ class Messages::MentionService
     end
   end
 
+  def self_mention?(user_id)
+    message.sender_type == 'User' && user_id.to_i == message.sender_id
+  end
+
   def add_mentioned_users_as_participants(validated_mentioned_ids)
     validated_mentioned_ids.each do |user_id|
       message.conversation.conversation_participants.find_or_create_by(user_id: user_id)