ssh: drop traffic on undecided channels

A channel in the mux's chanList is not usable until it is established:
an outbound channel has no confirmed remote id until the peer's open
confirmation, and an inbound channel is not serviced by the application
until it is accepted. handlePacket processed any channel message on it,
so a misbehaving peer could flood channel requests and block the mux
read loop on the send to incomingRequests, deadlocking the connection,
or close an outbound channel before confirming it, making the victim
tear down a half-initialized channel and emit a close for remote id 0,
an unrelated channel of the peer.

No such packet can be legitimate: the peer learns an inbound channel's
local id only from the confirmation we have not sent yet, and on an
outbound channel RFC 4254 lets it answer the open request only with a
confirmation or a failure.

Add an established flag, set when the channel becomes usable: for an
outbound channel when the open response is received, for an inbound
channel by Accept before the confirmation is sent. Until then
handlePacket drops every packet other than the open response. The flag
is separate from decided, which Reject also sets: a rejected channel is
decided but must never carry traffic.

Fixes CVE-2026-78662
Fixes golang/go#81316

Change-Id: Ib0983bb216a49808a2db1f4a4d92ee9fe38a3c51
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/826504
Auto-Submit: Gopher Robot <gobot@golang.org>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>
Reviewed-by: Neal Patel <nealpatel@google.com>
Reviewed-by: Nicholas Husin <husin@google.com>