[ty] Preserve non-final types in Hashable unions (#26039)
## Summary
In https://github.com/astral-sh/ty/issues/1162#issuecomment-4343373191,
we decided to stop simplifying `Hashable | C` to `Hashable` when `C` is
non-final. A subclass can disable hashing even when its base class
satisfies `Hashable`, so the existing normalization can discard a
meaningful union arm:
```python
from collections.abc import Hashable, Sequence
def f(value: Hashable | Sequence[Hashable]) -> None:
...
```
This recognizes the standard-library protocol through both
`typing.Hashable` and `collections.abc.Hashable`, and preserves the
non-final arm while retaining the existing simplification for final
classes and `object`.
This PR uses a relatively narrow / conservative model for determining
hashability; https://github.com/astral-sh/ruff/pull/26040 is more
expansive, but I think it deserves to be reviewed separately.
Closes https://github.com/astral-sh/ty/issues/1162.
