Support narrowing generic upper bounds

### Summary

No error should be emitted on this snippet:

```py
class A:
    pass

class B:
    x: int = 0

def _[C: A | B](c: C):
    if isinstance(c, B):
        reveal_type(c)
        c.x = 42
```

More discussion here: https://github.com/astral-sh/ruff/pull/18527#discussion_r2142659512

Example: https://play.ty.dev/62d91bdb-cbf1-4758-8f2c-6a26722c9f47