[ty] Contextually infer custom `__setattr__` assignments (#27015)
## Summary
Prior to this change, we inferred the right-hand side of an unresolved
instance-attribute assignment before checking a custom `__setattr__`.
That discarded the setter's expected type, leaving lambda parameters as
`Unknown` and rejecting otherwise-valid `TypedDict` literals:
```py
class Payload(TypedDict):
value: int
class Custom:
def __setattr__(self, name: str, value: Callable[[int], int] | Payload) -> None: ...
instance = Custom()
instance.callback = lambda value: value.missing # Previously: no diagnostic
instance.payload = {"value": 1} # Previously: unresolved-attribute
```
We now contextually infer unresolved instance assignments against the
custom setter, so the lambda parameter is correctly inferred as `int`
and the `TypedDict` literal is accepted. Explicit attributes and
terminal setters retain their existing behavior.
