x/crypto/ssh: prevent DoS on deadlocked established channel #81317

Previously, after a channel has been established, a
malicious peer could send crafted messages that would
deadlock the entire connection.

Now, we handle all RFC 4254 channel messages; global
requests are handled explicitly. Then, treat all other
messages as a protocol error and tear the connection
down instead of buffering and blocking.

Thanks to Will Mortensen for reporting this issue.

This is CVE-2026-56855 and Go issue https://go.dev/issue/81317.

This was originally a PRIVATE tracked in http://b/553514673.