[ty] dynamic class attributes are not instance attributes (#25678)
## Summary
Values from a dynamic class namespace are class attributes, not direct
instance members; treat them that way. This allows them to use the
descriptor protocol, matching runtime behavior. It also avoids the
members of a dynamically-created metaclass being treated as existing on
the class using that metaclass.
Closes https://github.com/astral-sh/ty/issues/3671
## Testing
Added mdtests.
