Function generate_getter_body 

fn generate_getter_body(
    struct_name: &Ident,
    slot: &SidecarSlot,
    _prot_index_lit: &LitInt,
) -> TokenStream

Generate the token stream for a sidecar getter function body.

Reads the field value from the Rust struct (accessed via the external pointer address) and converts it to an R SEXP. The conversion strategy depends on the slot kind:

• RawSexp: returns the SEXP field directly.
• Scalar kinds: wraps in Rf_Scalar* for zero-overhead conversion.
• Conversion: clones the value and calls IntoR::into_sexp.

The body runs inside with_r_unwind_protect (see the emission site in generate_sidecar_accessors); a non-external-pointer argument, a null pointer address, or a wrong stored type panics with the same expected ExternalPtr<T> message the main class-method path uses. The panic is transported as a tagged condition and re-raised by the R wrapper.