Function resolve_zone 

pub fn resolve_zone(
    annotations: &BTreeMap<String, String>,
    default_zone: Option<&str>,
) -> Option<String>

Resolves the DNS zone for an Ingress, in priority order:

1. bindy.firestoned.io/zone annotation — per-Ingress explicit override
2. default_zone — operator-configured default zone (e.g. "example.com")

Returns None if neither is available. When None, Scout logs a warning and skips the Ingress.