/**
 * Convert an ISO 8601 zoned datetime string to a UTC Instant string (ISO).
 *
 * - Uses Temporal.ZonedDateTime.toInstant to convert.
 * - Returns "" for invalid input.
 *
 * @param value zoned ISO 8601 datetime string
 * @returns UTC Instant ISO string or "" when invalid
 *
 * @example convertZonedToUtc("2024-02-29T12:34:56.789+00:00[UTC]") // "2024-02-29T12:34:56.789Z"
 * @example convertZonedToUtc("invalid") // ""
 */
export declare function convertZonedToUtc(value: string): string;