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)

spec/services/messages/mention_service_spec.rb

@@ -165,6 +165,36 @@ describe Messages::MentionService do
       end
     end
 
+    context 'when the message sender mentions themselves' do
+      it 'skips the sender notification while notifying other mentioned users' do
+        message = build(
+          :message,
+          conversation: conversation,
+          account: account,
+          content: "hey (mention://user/#{first_agent.id}/#{first_agent.name}) and (mention://user/#{second_agent.id}/#{second_agent.name})",
+          private: true,
+          sender: first_agent
+        )
+
+        described_class.new(message: message).perform
+
+        expect(NotificationBuilder).not_to have_received(:new).with(
+          notification_type: 'conversation_mention',
+          user: first_agent,
+          account: account,
+          primary_actor: message.conversation,
+          secondary_actor: message
+        )
+        expect(NotificationBuilder).to have_received(:new).with(
+          notification_type: 'conversation_mention',
+          user: second_agent,
+          account: account,
+          primary_actor: message.conversation,
+          secondary_actor: message
+        )
+      end
+    end
+
     context 'when mentioned user is not an inbox member' do
       let!(:non_member_user) { create(:user, account: account) }