[ty] Improve `isinstance()` reachability analysis (#24077)
## Summary
Our reachability analysis for `isinstance()` constraints currently
doesn't match the sophistication of our type narrowing in this
situation. This was obscured in our tests by the fact that
`assert_never()`, `reveal_type` and `assert_type` calls can in fact
create their own reachability constraints!
`assert_type`, `reveal_type` and `assert_never` are all generic
functions that return an object of the same type as the object passed
into the function. Following
https://github.com/astral-sh/ruff/pull/23419, that means that if you
pass an object inferred as having the type `Never` into one of these
functions, all code following that call will be inferred as being
unreachable. In other words, this test was passing entirely by accident,
because of additional reachability constraints introduced by the
`reveal-type` and `assert_never` calls!
https://github.com/astral-sh/ruff/blob/1aabbbc89160251cf13d3282a4963db84419a3bb/crates/ty_python_semantic/resources/mdtest/exhaustiveness_checking.md?plain=1#L470-L477
Remove the `else` branch from that test, and you get a false-positive
`invalid-return-type` diagnostic complaining that the function can
implicitly return `None` -- which is clearly not the case:
```py
def h[T: int | str](x: T) -> T:
if isinstance(x, int):
return x
elif isinstance(x, str):
return x
```
The fix is to adapt our `isinstance()` special casing so that it has the
same sophistication as our type narrowing machinery here, where we
understand that `T & ~U & ~S` must resolve to `Never` if `T` is a type
variable bound to the union `S | U`. In a similar way, an `else` branch
(explicit or implicit) can never be taken in either of the `h` functions
above.
## Test Plan
Added mdtests that fail on `main`
