Function ceil_whole_sec_from_parts 

pub(crate) fn ceil_whole_sec_from_parts(
    secs: i64,
    has_fraction: bool,
) -> Result<i64>

Pure ceiling computation factored out of ceil_to_whole_sec so the overflow path is testable: chrono::DateTime<Utc> cannot represent instants near i64::MAX seconds, but the arithmetic below guards it anyway.