Function normalize_iso_datetime 

pub fn normalize_iso_datetime(s: &str) -> Option<String>

Normalizes a date or datetime string to an ISO-8601 datetime with an explicit UTC offset (Z).

• Bare date YYYY-MM-DD -> YYYY-MM-DDT00:00:00Z
• Datetime without offset YYYY-MM-DDTHH:MM:SS -> YYYY-MM-DDTHH:MM:SSZ
• Datetime with space separator YYYY-MM-DD HH:MM:SS -> YYYY-MM-DDTHH:MM:SSZ
• Already valid datetime with offset -> canonical formatted string
• Unparseable string -> None