[ty] Do not top-materialize (user) `TypeIs` return types (#26864)
## Summary
We currently top-materialize `TypeIs` types to work around the fact that
typeshed annotates some standard library functions like `isawaitable`
with a gradual `TypeIs[Awaitable[Any]]` return type. This is problematic
when taken verbatim, since that instructs us to intersect with a gradual
`Awaitable[Any]` type instead of truly selecting *all* awaitables
(`Awaitable[object]`). For example, narrowing `Item | Awaitable[Item]`
with a final `Item` class using `isawaitable` would result in
`Awaitable[Item] & Awaitable[Any] = Awaitable[Item & Any]`, instead of
just `Awaitable[Item]`.
However, the current behavior is problematic for user-defined `TypeIs`
functions, since we don't respect their declared return type.
Here, we fix this by patching typeshed while dropping the
top-materialization. This leads to an unchanged behavior for
standard-library functions like `isawaitable`, `iscallable`, etc, but
respects users annotations for custom `TypeIs` functions:
```py
def is_list(arg: object) -> TypeIs[list[Any]]:
return isinstance(arg, list)
def _(x: object):
if is_list(x):
reveal_type(x) # list[Any], previously: Top[list[Any]]
```
relates to https://github.com/astral-sh/ty/issues/3375
## Ecosystem report
Looks mostly good: Removed false positives, new unused ignore comment
diagnostics
Some expected problems, e.g. with this user-defined function in
beartype, leading to unfortunate types like `((...) -> Unknown) &
~((...) -> Unknown)`, which are technically correct (and could maybe be
simplified), but certainly unintended:
```py
def is_callable_like(value: Any) -> TypeIs[Callable]:
return callable(value)
```
## Conformance results
- One false positive removed, which is expected.
- One new false positive, which looks like it's due to a generic solver
gap.
## Test Plan
Adapted tests
