When GNS3 server is configured to listen on 0.0.0.0 (all interfaces),
_server_url() was hardcoding 127.0.0.1, making the WebSocket console
URL unreachable from remote MCP clients.
Use the UDP connect trick (connect to 8.8.8.8:80 without sending data)
to discover the default route interface IP, which is the address remote
clients can actually reach.