Function make_keyed_map_variant_deser_code

Full deserialization body for a collapsed map-rep group-choice arm that carries a fixed key: len-check, key read+verify, value read, final len-check, and the variant constructor. The map holds exactly one pair (key + value), so the length check is `Fixed(1)`. Used by both enum dispatch paths (type-match arm body / brute-force closure body).

(
    gen_scope: &mut GenerationScope,
    types: &IntermediateTypes,
    name: &RustIdent,
    variant: &EnumVariant,
    key: &FixedValue,
    enum_gen_info: &EnumVariantInRust,
    cli: &Cli,
)