Function prepare 

pub async fn prepare<R>(
    pool: &SqlitePool,
    args: R,
    app_config: &BhtuneConfig,
) -> Result<PreparedTune>
where
    R: TryInto<ValidatedTuneRequest>,
    R::Error: Error + Send + Sync + 'static,

Validates a tune request and prepares the driver and persisted run record.

The unattended-write guard and timing/config validation run before driver I/O. The caller’s request is snapshotted before effective connection defaults are resolved, and the resolved connection and tuning context are persisted before polling starts.