The CBOR implementation is not properly handling recursive data structures by failing to apply Box to union variants where required. This results in uncompilable Rust code for recursive types.

Issue Details

Using the following Smithy model with recursive type references:

@rpcv2Cbor
service SampleService {
    operations: [
        GetRelatedProduct
    ]
}

operation GetRelatedProduct {
    input := {
        product: ProductListing
    }
    output := {
        product: ProductListing
    }
}

union ProductListing {
    Related: RelatedProduct
}

structure RelatedProduct {
    name: String,
    relatedTo: ProductListing
}

Current Output

The union deserializer currently generates code without boxing the recursive type:

Ok(match decoder.str()?.as_ref() {
    "Related" => crate::types::ProductListing::Related(crate::protocol_serde::shape_related_product::de_related_product(decoder)?),
    _ => {
        decoder.skip()?;
        crate::types::ProductListing::Unknown
    }
})

Expected Output

The deserializer should box recursive variants to prevent infinite size errors:

Ok(match decoder.str()?.as_ref() {
    "Related" => crate::types::ProductListing::Related(
        Box::new(crate::protocol_serde::shape_related_product::de_related_product(decoder)?)
    ),
    _ => {
        decoder.skip()?;
        crate::types::ProductListing::Unknown
    }
})

This implementation correctly uses Box to handle the recursive relationship between ProductListing and RelatedProduct.