Function get_unique_edges_for_data 

pub async fn get_unique_edges_for_data(
    db: &DatabaseConnection,
    data_id: Uuid,
    dataset_id: Uuid,
) -> Result<Vec<GraphEdge>, DatabaseError>

Return edges belonging to (data_id, dataset_id) whose slug does NOT appear in any other row within the same dataset with a different data_id.