Function gen_opaque_constructor 

pub fn gen_opaque_constructor(
    ctor: &ClientConstructorConfig,
    type_name: &str,
    core_import: &str,
    constructor_attr: &str,
) -> String

Generate a custom opaque-handle constructor from a ClientConstructorConfig.

Emits a pub fn new(params…) -> Result<Self, ErrType> method body suitable for inclusion inside an impl TypeName { … } block.

• constructor_attr — optional attribute line placed immediately before the pub fn new line (e.g. "#[new]" for PyO3, "#[napi(constructor)]" for NAPI-RS, or "" to emit no attribute).