[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/resources/mdtest/generics/legacy/functions.md

@@ -530,3 +530,17 @@ age, name = team.employees[0]
 reveal_type(age)  # revealed: Age
 reveal_type(name)  # revealed: Name
 ```
+
+## `~T` is never assignable to `T`
+
+```py
+from typing import TypeVar
+from ty_extensions import Not
+
+T = TypeVar("T")
+
+def f(x: T, y: Not[T]) -> T:
+    x = y  # error: [invalid-assignment]
+    y = x  # error: [invalid-assignment]
+    return x
+```

crates/ty_python_semantic/resources/mdtest/generics/pep695/functions.md

@@ -543,3 +543,14 @@ def _(x: int):
 
     reveal_type(C().implicit_self(x))  # revealed: tuple[C, int]
 ```
+
+## `~T` is never assignable to `T`
+
+```py
+from ty_extensions import Not
+
+def f[T](x: T, y: Not[T]) -> T:
+    x = y  # error: [invalid-assignment]
+    y = x  # error: [invalid-assignment]
+    return x
+```