Enhancement: Include Notification Messages for 'Assigned' and 'Mentions' Similar to 'Reported' and 'Watching'

Add similar notification messages for 'Assigned' and 'Mentions', similar to those for 'Reported' and 'Watching', like 'You have been assigned this task' or 'You were mentioned in this task'.

<img width="961" height="774" alt="Image" src="https://github.com/user-attachments/assets/bad35e89-4ffc-4da0-bc4d-d99822af63f6" />