Function est_error_response 

pub fn est_error_response(status: StatusCode, detail: &str) -> Response

Build an EST error response per RFC 7030 §4.2.3.

EST error responses use HTTP status codes as the primary error indicator. For enrollment failures, the server MAY return a CMC Full PKI Response body with detailed error information.

For now, this returns a text/plain body with the error detail. A future enhancement will return a proper CMC error body for 4xx responses on enrollment endpoints.