[ty] Check inherited attribute conflicts (#28559)
## Summary
We now report incompatible attributes inherited from different bases,
even when the subclass does not override them:
```python
class Integer:
value: int
class String:
value: str
class Combined(Integer, String): ... # invalid-attribute-override
```
We check the member selected by the MRO against the base declarations,
including properties and annotations on `self`. Conflicts already
present in a parent are not reported again.
Part of https://github.com/astral-sh/ty/issues/2158.
