Bogus SSHFS health check

**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.

https://github.com/canonical/multipass/blob/f4542c359969da40ec92dadb4a2592eea2f38337/src/sshfs_mount/sftp_server.cpp#L313
https://github.com/canonical/multipass/blob/b0ed7e3e077a2e1d596a2d57136a6dd4a8b1458c/src/ssh/ssh_process.cpp#L48