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.
ginuerzh