GHSA-M65R-RPRJ-R5RG Russh: Channel-scoped server callbacks can be reached without an open channel

There is a server-side channel state issue in russh. After a client is authenticated, russh can dispatch channel-scoped handler callbacks for recipient channel IDs that were never opened or confirmed. In the strongest reproduced case, the client does not send SSHMSGCHANNELOPEN at all. It...