[ty] ~T should never be assignable to T (#20606)

## Summary

Currently we do not emit an error on this code:

```py
from ty_extensions import Not

def f[T](x: T, y: Not[T]) -> T:
    x = y
    return x
```

But we should do! `~T` should never be assignable to `T`.

This fixes a small regression introduced in
14fe1228e7 (diff-8049ab5af787dba29daa389bbe2b691560c15461ef536f122b1beab112a4b48aR1443-R1446),
where a branch that previously returned `false` was replaced with a
branch that returns `C::always_satisfiable` -- the opposite of what it
used to be! The regression occurred because we didn't have any tests for
this -- so I added some tests in this PR that fail on `main`. I only
spotted the problem because I was going through the code of
`has_relation_to_impl` with a fine toothcomb for
https://github.com/astral-sh/ruff/pull/20602 😄

crates/ty_python_semantic/src/types.rs

@@ -1567,7 +1567,7 @@ impl<'db> Type<'db> {
             (Type::Intersection(intersection), Type::NonInferableTypeVar(_))
                 if intersection.negative(db).contains(&target) =>
             {
-                ConstraintSet::from(true)
+                ConstraintSet::from(false)
             }
 
             // Two identical typevars must always solve to the same type, so they are always