Function validate_and_prepare_timer 

pub fn validate_and_prepare_timer(
    name: &str,
    interval_ns: u64,
    start_time_ns: Option<UnixNanos>,
    stop_time_ns: Option<UnixNanos>,
    allow_past: Option<bool>,
    fire_immediately: Option<bool>,
    ts_now: UnixNanos,
) -> Result<(Ustr, UnixNanos, Option<UnixNanos>, bool, bool)>

Validates and prepares parameters for setting a timer.

Handles name and interval validation, default value unwrapping, start time normalization, and stop time validation.

Errors

Returns an error if name is invalid, interval is not positive, or stop time validation fails.