ParseAuthorizedKeysFile looped over ssh.ParseAuthorizedKey, but x/crypto's
parser splits on '\n' and returns rest='\n' for a trailing newline. The next
iteration called ParseAuthorizedKey('\n'), which skips whitespace and returns
'ssh: no key found', aborting the whole parse. Any normal authorized_keys file
(ends with newline, or has comments/blank lines) failed despite being valid.
Break the loop once rest is exhausted so all keys are parsed.
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