/**
 * Pull the SERVER's actual error message out of a failed request.
 *
 * NestJS returns `{ message: string | string[] }`; a class-validator failure is
 * the array form (joined here). Falls back to the error's own `.message` — the
 * axios response interceptor (see `auth/http.ts`) already rewrites that to the
 * server message, so even code paths that never see the raw response body get
 * the real cause — then to the caller's fallback string.
 */
export const extractErrorMessage = (err: unknown, fallback: string): string => {
  const msg = (err as { response?: { data?: { message?: string | string[] } } })
    ?.response?.data?.message;
  if (Array.isArray(msg)) return msg.filter(Boolean).join(", ");
  if (typeof msg === "string" && msg) return msg;
  if (err instanceof Error && err.message) return err.message;
  return fallback;
};