Bug description

After upgrading jupyter-server-proxy from 4.1.2 to 4.4.0 and switching the config from 'type': 'tcp' to 'raw_socket_proxy': True, SSH connections through Jupyter are disconnected after about 1 minute of inactivity.

How to reproduce

Use this server proxy config:

c.ServerProxy.servers = {
    'sshd': {
        'raw_socket_proxy': True,
        'port': 8888,
        'launcher_entry': {'enabled': False},
    }
}

Connect via SSH: ssh -p 8888 user@localhost
Keep the session idle for ~60 seconds.

Expected behaviour

The SSH connection should stay alive during inactivity, just like it did with the old 'type': 'tcp' mode.

c.ServerProxy.servers = {
    'sshd': {
        'type': 'tcp',
        'port': 8888,
        'launcher_entry': {'enabled': False},
    }
}

Actual behaviour

Connection is dropped with:

websocket: close 1006 (abnormal closure): unexpected EOF
going to close all connection
Connection closed by UNKNOWN port 65535

Your personal set up

Client: macOS (arm64, Apple Silicon)
Server: CentOS 7 (x86_64 Linux)
jupyter-server-proxy: 4.4.0 (worked fine in 4.1.2)