Implements proper socket-level health checking for gateway websocket connections:
- Add is_connection_alive() method to GatewayTransceiver trait
- Implement socket-level health check using TcpStream::peek() on Unix systems
- Add is_gateway_connection_alive() method to MixnetClient for easy access
- Update network monitor to use connection health checks before sending
- Fix axum router state configuration in network monitor
The health check performs actual OS-level socket validation rather than just
checking file descriptor existence, providing reliable connection status.
durch
b45bb9e7e9