- Implements a toggleable shell mode, removing the need to prefix every command with `!`.
- Users can now enter and exit shell mode by typing `!` as the first character in an empty input prompt.
- The input prompt visually indicates active shell mode with a distinct color and `! ` prefix.
- Shell command history items (`user_shell`) are now visually differentiated from regular user messages.
- This provides a cleaner and more streamlined user experience for frequent shell interactions.
Fixes https://b.corp.google.com/issues/418509745
