fix(agentchat_fastapi): handle WebSocket disconnect gracefully in app_team.py

- Add specific WebSocketDisconnect exception handling in _user_input function
- Handle client disconnects during message processing without RuntimeError
- Prevent attempts to send messages to closed WebSocket connections
- Add graceful cleanup when clients disconnect while waiting for input

Fixes #6280: Heavy use of broad Exceptions leads to incorrect error handling

The previous implementation caught WebSocketDisconnect with broad Exception
handlers and re-raised as RuntimeError, preventing graceful disconnect handling.
This fix allows proper exception propagation and clean connection termination.

Author: tejas-dharani