Function create_engine 

pub fn create_engine(
    args: MockEngineArgs,
    dp_rank: u32,
    output_tx: Option<UnboundedSender<Vec<OutputSignal>>>,
    kv_event_publishers: KvEventPublishers,
    cancellation_token: Option<CancellationToken>,
    fpm_publisher: FpmPublisher,
) -> Box<dyn SchedulerHandle>

Create a scheduler for the configured engine type.

Returns a boxed SchedulerHandle that the engine wrapper can use without knowing which backend is running underneath.