Function validate_at_time 

async fn validate_at_time(
    client: &Client,
    test: &UnitTest,
) -> Result<Result<(), InvalidAtTimeError>, CliError>

Pre-validates optional AT TIME test expressions against mz_timestamp casting.

Returns Ok(Ok(())) when valid, Ok(Err(InvalidAtTimeError)) on validation failure, and Err(CliError) on connection errors.