ssh: Connection Deadlock via Request Flood on an Unconfirmed Channel

A malicious SSH peer can flood the incomingRequests buffer of a channel that has not yet been confirmed. Because the channel is not usable, Go’s golang.org/x/crypto/ssh multiplexer blocks trying to process those packets and the entire connection deadlocks, a denial of service. The fix tracks an atomic established state per channel and discards any packet other than a confirmation or failure until the channel is usable, so the mux never blocks on an unconfirmed channel.