Let instant_remote_process throw on failed SSH/docker commands so the
real error (e.g. a missing coolify-sentinel container or an unreadable
SSH key) is reported. A null result, such as a literal "null" body, is
turned into an empty string, which guard() rejects as an invalid response.
Run curl with -sS so progress output stays out of the output while
errors are still reported.
Add tests for a failed docker exec and a literal null body.