Function from_nested 

pub async fn from_nested(
    consumer: &impl SubstraitConsumer,
    nested: &Nested,
    input_schema: &DFSchema,
) -> Result<Expr, DataFusionError>

Converts a Substrait [Nested] expression into a DataFusion [Expr].

Substrait Nested expressions represent complex type constructors (list, struct, map) where elements are full expressions rather than just literals. This is used by producers that emit Nested { list: ... } for array construction, as opposed to Literal { list: ... } which only supports scalar values.