Get the current date and time.

Returns structured JSON with the current datetime, including:

• ISO 8601 datetime string

• Date (YYYY-MM-DD)

• Time (HH:MM:SS)

• Day of week in English and Persian

• IANA timezone name

• UTC offset

• Unix timestamp

• Jalali (Solar Hijri) date in numeric and Persian formats

Args: timezone: Optional IANA timezone name (e.g., "Asia/Tehran", "Europe/Berlin", "America/New_York", "UTC"). If not provided, uses system local timezone.

Returns: JSON string with datetime information, or error message for invalid timezone.

Examples: - get_current_datetime() -> Current time in system timezone - get_current_datetime("Asia/Tehran") -> Current time in Tehran - get_current_datetime("UTC") -> Current time in UTC