[arg-type] error in assignment of generic with unions

```py
from typing import Callable

def f[T](x: Callable[[], T] | Callable[[list[object]], T]) -> T: ...

a: list[int] | list[str] = f(list) # Argument 1 to "f" has incompatible type "type[list[_T]]"; expected "() -> Never | (list[object]) -> Never" [arg-type]
```

Start by running the provided Python reproducer and examining how the checker handles generic inference for the union of Callable signatures in this assignment. Trace the source path that emits the [arg-type] diagnostic; done means the example no longer reports the incompatible argument while preserving correct type checking.

Written by the indexing model from the issue text.