/**
 * Parse a duration into milliseconds.
 *
 * Accepts:
 *   - Number → treated as milliseconds (must be positive integer)
 *   - String  → '500ms', '30s', '15m', '1h', '7d', '2w' (case-insensitive)
 *
 * Rejects zero, negatives, non-finite, unknown suffixes, and empty strings.
 *
 * @param {string | number} input
 * @param {string} [field='window'] — field name used in error messages
 * @returns {number} milliseconds
 */
export function parseDuration(input: string | number, field?: string): number;