/**
 * Return the number of days in the month containing `value`.
 *
 * - 28–31 depending on the month; February varies by leap year.
 * - Returns null on invalid input.
 *
 * @param value ISO PlainDate string
 * @returns number of days in the month (28-31), or null on invalid input
 *
 * @example getDaysInMonth("2024-02-15") // 29 (leap year)
 * @example getDaysInMonth("2023-02-15") // 28
 * @example getDaysInMonth("2024-04-01") // 30
 * @example getDaysInMonth("2024-01-01") // 31
 * @example getDaysInMonth("invalid") // null
 */
export declare function getDaysInMonth(value: string): number | null;