Bogus SSHFS health check

[ricab]

Describe the bug
The SftpServer calls release_channel() on the SSHFS SSHProcess, which nulls the channel field. But it later asks the process for the exit code, which internally registers an ExitStatusCallback with a null channel. An SSH_ERROR code is returned, but never handled.

The process's exit code can never be observed and every health check times out silently. Recovery is driven by EOF alone.

multipass/src/sshfs_mount/sftp_server.cpp

Line 313 in f4542c3

sftp_server_session{make_sftp_session(ssh_session, sshfs_process->release_channel())},

multipass/src/ssh/ssh_process.cpp

Line 48 in b0ed7e3

ssh_add_channel_callbacks(channel, &cb);