russh: Client-side channel-scoped Handler callbacks fire for channel IDs the client never opened

Summary CVE-2026-68930 was fixed by adding Session::isestablishedchannel in russh/src/server/encrypted.rs, which gates every channel-scoped SERVER-side message CHANNELREQUEST, CHANNELDATA, CHANNELEOF, CHANNELCLOSE, CHANNELWINDOWADJUST, CHANNELEXTENDEDDATA on enc.channels.get&channel.issomeand|c|...