[ENHANCEMENT] Add @mention tagging in ticket comments

[BenGWeeks]

Description

Add the ability to @mention and tag users in ticket replies/comments, similar to how GitHub, Slack, and other collaboration tools work.

Expected Behavior

• When typing @ in a comment, show a dropdown of available users
• Autocomplete user names as you type
• Mentioned users should be visually highlighted in the comment
• (Future) Mentioned users could receive notifications

Use Cases

• Tag team members for attention on specific tickets
• Assign follow-up tasks within comments
• Loop in subject matter experts for complex issues

Technical Considerations

• Use existing /api/devops/users or team members data for autocomplete
• Consider debouncing the user search
• Store mentions as plain text (e.g., @username) for Azure DevOps compatibility