Support failable mark ( ? ) in function return type annotations and enforce consistency rules

Support failable mark ( ? ) in function return type annotations and enforce consistency rules: a task in LegoFlow-SWE (Harbor dataset). 1. Failable return type annotation A function may declare its return type with a trailing ? to mark it as failable: fun test(): Num? { … } fun safe division(a…

The task

**1. Failable return type annotation** A function may declare its return type with a trailing `?` to mark it as failable: ``` fun test(): Num? { … } fun safe_division(a: Num, b: Num): Num? { … } ```

Part of Lego-X/LegoFlow-SWE.