Function acme_cert_present 

pub fn acme_cert_present(
    path: &Path,
    resolver: &str,
    hostname: &str,
) -> Result<bool>

Returns true if acme.json at path contains a certificate entry whose domain.main or domain.sans matches hostname under the given resolver.

Returns false (not an error) when:

• The file does not exist yet (Traefik hasn’t started the challenge)
• The resolver section is absent
• The certificates array is empty

Returns Err only for genuine I/O or JSON parse failures.