[ty] Generalize nested types in inferred return types - #29488

Summary

Generalize/promote types in inferred return types that refer to local nested definitions.

For example, consider this unannotated decorator:

 def register(event: str):
     def wrapper(func):
         # ...
         return func

     return wrapper

We do not want to infer def wrapper(func) -> Unknown as the return type of register. This would not just expose internal details, it would also incorrectly suggest that each call to register returns the same function object. Instead, we generalize to an anonymous (func) -> Unknown callable.

For nested classes, we cannot easily do the same, so we return Unknown instead. In the future, to improve this, we might want to synthesize a protocol that has the same attributes and methods as the nested class.

Test strategy

Updated existing mdtests