Function s3_xml_error_response_with_fields 

pub fn s3_xml_error_response_with_fields(
    status: StatusCode,
    code: &str,
    message: &str,
    request_id: &str,
    extra_fields: &[(String, String)],
) -> (StatusCode, String, Bytes)

Build an S3-style XML error response with additional fields (e.g., BucketName, Key).