/**
* Read the server-issued request id from response headers. OpenAI surfaces
* `x-request-id`, Anthropic uses `request-id`, many gateways use
* `x-correlation-id` or `cf-ray`. Caller supplies the candidate list.
*
* Returns the first non-empty header value, or undefined.
*/
export declare function readRequestId(headers: Headers, candidates: readonly string[]): string | undefined;