[ty] Use declared attribute types as type context (#21143)

## Summary

For example:
```py
class X:
    x: list[int | str]

def _(x: X):
    x.x = [1]
```

Resolves https://github.com/astral-sh/ty/issues/1375.

crates/ty_python_semantic/resources/mdtest/call/union.md

@@ -281,46 +281,3 @@ def _(flag: bool):
     # we currently consider `TypedDict` instances to be subtypes of `dict`
     f({"y": 1})
 ```
-
-Diagnostics unrelated to the type-context are only reported once:
-
-`expression.py`:
-
-```py
-def f[T](x: T) -> list[T]:
-    return [x]
-
-def a(x: list[bool], y: list[bool]): ...
-def b(x: list[int], y: list[int]): ...
-def c(x: list[int], y: list[int]): ...
-def _(x: int):
-    if x == 0:
-        y = a
-    elif x == 1:
-        y = b
-    else:
-        y = c
-
-    if x == 0:
-        z = True
-
-    y(f(True), [True])
-
-    # error: [possibly-unresolved-reference] "Name `z` used when possibly not defined"
-    y(f(True), [z])
-```
-
-`standalone_expression.py`:
-
-```py
-def f(_: str): ...
-def g(_: str): ...
-def _(a: object, b: object, flag: bool):
-    if flag:
-        x = f
-    else:
-        x = g
-
-    # error: [unsupported-operator] "Operator `>` is not supported for types `object` and `object`"
-    x(f"{'a' if a > b else 'b'}")
-```