`ContextDecorator` always returns a function

in contextlib.pyi it should be:
```py
class ContextDecorator:
def __call__(self, func: "(P) -> R") -> "def (P) -> R": ...
```

The requested annotation is in contextlib.pyi; start by reading the ContextDecorator definition and its __call__ signature. Update the return typing to preserve the decorated function's parameter and return types, then confirm the stub check passes. Done means __call__ no longer always reports a generic function return.

Written by the indexing model from the issue text.