Function extract_json_field 

pub(super) fn extract_json_field(
    secret_string: &str,
    json_key: &str,
    secret_name: &str,
) -> Result<String, String>

Extract a top-level string field from a JSON secret.

Returns the field’s string content on success. On failure, returns a reason string suitable for SecretResolveError::ResolutionFailed.