fix(dialer): evict dead SSHD sessions before reuse to prevent channel-open failure (#382)

When an SSH session behind an SSHD dialer dies silently (NAT timeout,
server disconnect, idle TCP drop), IsClosed() may still return false
because the health-check goroutine hasn't detected the failure yet.
Returning this dead cached session to the connector causes:

  ssh: unexpected packet in response to channel open:

Add a Ping(req)-based liveness check before returning a cached session.
If the ping fails, evict the session so the next Dial rebuilds a fresh
connection. This matches the dead-session eviction pattern already used
by the SSH, KCP, QUIC, and ICMP dialers.

Also export the ping-as-health-check as Session.Ping(timeout) so it can
be used by external health probes. Retain the existing ping() helper
unmodified.

dialer/sshd/dialer.go

@@ -63,6 +63,18 @@ func (d *sshdDialer) Dial(ctx context.Context, addr string, opts ...dialer.DialO
 		delete(d.sessions, addr) // session is dead
 		ok = false
 	}
+	if ok {
+		// Health check: verify the cached session is still alive before
+		// returning it. A dead-but-not-yet-detected session (e.g. idle
+		// connection silently dropped by NAT) would cause "ssh: unexpected
+		// packet in response to channel open" later in the connector.
+		if err := session.Ping(d.md.keepaliveTimeout); err != nil {
+			d.options.Logger.Debugf("sshd session ping failed: %v, recreating", err)
+			session.Close()
+			delete(d.sessions, addr)
+			ok = false
+		}
+	}
 	if !ok {
 		var options dialer.DialOptions
 		for _, opt := range opts {