[ty] Infer `bool` for `not` applied to dynamic values (#25445)

## Summary

Prior to this change, we treated all unary operations applied to a
dynamic value as producing the same dynamic type. This is appropriate
for `+`, `-`, and `~`, whose dunder implementations may return arbitrary
values, but not for `not`: Python always produces a `bool` after
evaluating its operand's truthiness.

```python
from typing import Any, reveal_type

def check(value: Any) -> None:
    result = reveal_type(not value)  # bool
    result.nonexistent()  # error: `bool` has no attribute `nonexistent`
```

We now allow `not` on dynamic operands to use the existing
truthiness-inference path, while retaining dynamic propagation for the
other unary operators. This restores downstream checking of the
resulting boolean value.

Closes https://github.com/astral-sh/ty/issues/3572.