Check Collexus Sftp

Probe the Collexus SFTP server and report what came back.

The creditor's firewall admits only the deployment's NAT egress addresses, so this has to run from inside the stack — and the image carries no ssh client, so an admin route is the way to run it. It answers cleanly on a deployment that has no Collexus configuration at all.

With no COLLEXUS_SFTP_HOST_KEY set the check runs in discovery mode: it reports the server's fingerprint and the key line to pin, having authenticated nothing and transferred nothing. Once pinned, a host key that does not match aborts the connection and the check says so.