ssh: prevent memory leak when rejecting channels

When a server rejects an incoming channel request via
NewChannel.Reject, the channel is left in the multiplexer's
channel list. Because the channel is never explicitly removed or
closed, its internal buffers and sync primitives remain allocated
for the lifetime of the SSH connection.

A malicious client could exploit this behavior by repeatedly
requesting to open channels that are destined to be rejected,
causing unbounded memory growth and potentially leading to a
Denial of Service (DoS) via resource exhaustion.

This change fixes the leak by calling ch.mux.chanList.remove
within the Reject method, removing the channel from the list and allowing the
garbage collector to reclaim the associated memory immediately.

Fixes golang/go#35127
Fixes CVE-2026-3982

Change-Id: Iaa177f5dfd151812dd404e528a4a1c77527a0e29
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/781320
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: Nicholas Husin <nsh@golang.org>
Reviewed-by: Nicholas Husin <husin@google.com>