x/crypto/ssh: prevent DoS on deadlocked undecided channel #81316

Previously, a channel registered in the mux's chanList is
not usable until it is established. A malicious peer was
able flood the channel's incomingRequests, deadlocking the
entire connection.

Now, we add an atomic established state, set when a channel
becomes usable. Until such a time, handlePacket drops every
packet other than the open confirmation/failure, without
blocking and without tearing down the connection.

Thanks to Will Mortensen for reporting this issue.

This is CVE-2026-78662 and Go issue https://go.dev/issue/81316.

This was a PRIVATE originally tracked in http://b/553516417.