secureSocketProbe function
Whether the atServer at host:port completes a TLS handshake.
A lighter check than the default httpsProbe: it opens a secure socket and drops it, proving the port is listening and its certificate chain is acceptable, without asking the atServer to answer anything. It cannot tell a listening TLS terminator apart from a healthy atServer behind one.
Requires dart:io, which is why it lives here rather than in the main
barrel. Inject it as probeSocket where that trade is the one you want.
Implementation
Future<void> secureSocketProbe(String host, int port) async {
final socket = await SecureSocket.connect(host, port, timeout: probeTimeout);
socket.destroy();
}