Function dict_value_is_variant 

fn dict_value_is_variant(
    signature: Option<&str>,
    span: Span,
) -> Result<bool, Error>

Whether the dict’s value type is Variant (i.e. signature a{?v}).

Variant-typed values get wrapped/unwrapped via as_value; any other value type defers to the field type’s own Serialize/Deserialize.