Function verify_task_is_client_capable 

fn verify_task_is_client_capable(
    task_id: Uuid,
    exercise_type: &str,
    capable_slugs: &[String],
) -> Result<(), ControllerError>

Rejects a submission to a task whose exercise service cannot serve a native client.

The slide/task ids come from the request body, so without this a caller holding any task id could persist an answer to a service that will never understand it, and grading would fail deep inside that service instead of at the edge.