`T | None <= int | None` constraint wrongly solves T to `int | None`, should be `int`

[carljm]

from typing import Protocol

class Expected(Protocol):
    def value(self) -> int | None: ...

class Actual[T]:
    def value(self) -> T | None:
        return None

def make[T](_: list[T]) -> Actual[T]:
    return Actual()

def preserve[T: Expected](value: T) -> T:
    return value

def test() -> Actual[int]:
    return preserve(make([]))  # error: expected `Actual[int]`, found `Actual[int | None]`

This originally showed up with stdlib async-related types in #3583 (Generator.close() returns _ReturnTCo | None, which triggers this issue). We can easily work around it for that particular case by special-casing Generator, but the general bug in constraint solving remains.