feat(client): make request-id header more accessible (#462)

Author: stainless-app[bot]

src/error.ts

@@ -9,6 +9,8 @@ export class APIError extends AnthropicError {
   readonly headers: Headers | undefined;
   readonly error: Object | undefined;
 
+  readonly request_id: string | null | undefined;
+
   constructor(
     status: number | undefined,
     error: Object | undefined,
@@ -18,6 +20,7 @@ export class APIError extends AnthropicError {
     super(`${APIError.makeMessage(status, error, message)}`);
     this.status = status;
     this.headers = headers;
+    this.request_id = headers?.['request-id'];
     this.error = error;
   }