CVE-2026-42552: Flight vulnerable to sensitive information disclosure via default error handler

The default error handler Engine::_error() writes the full exception message, exception code, and stack trace (including absolute filesystem paths) directly into the HTTP 500 response, with no debug gating. Production deployments leak internal paths, any secret interpolated into an exception message, and full module structure — giving attackers primitives for chaining other weaknesses (LFI, path traversal).