Function resolve_api_key_with_fallback 

pub fn resolve_api_key_with_fallback(
    default_env: &str,
    fallback_env: &str,
    override_env: Option<&str>,
) -> Option<String>

Resolve an API key with a legacy fallback env var.

If override_env is set, it remains authoritative and no fallback is attempted. Without an override, default_env is checked first and fallback_env is accepted only when the default is unset.