Multiple fixes and UI improvements based on feedback

[daxian-dbw]

PR Summary

Multiple fixes and UI improvements based on feedback

1. Fix a bug when run @Azure <query> before the Azure agent starts a chat session.
2. Reset argument placeholder information when RefreshChatAsync is called on the Azure agent.
3. Update the initial agent selection message to let the user know they can switch to another agent later by @ tagging its name.
4. Update the read-line prompt to start with @, like @Azure> , so as to make the @ tagging more obvious.
5. Update the color used for /replace in the Run /replace to get assistance in placeholder replacement message to be more eye catching.