Function derive_record_name 

pub fn derive_record_name(host: &str, zone: &str) -> Result<String>

Derives the DNS record name from a hostname and zone.

• host.zone → host (e.g. "app.example.com" + "example.com" → "app")
• zone (apex) → "@"
• deep.sub.zone → "deep.sub"

Trailing dots on host are stripped before comparison.

Errors

Returns an error if host does not end with the zone suffix.