[ty] Support type context of union attribute assignments (#21170)

## Summary

Turns out this is easy to implement. Resolves
https://github.com/astral-sh/ty/issues/1375.

crates/ty_python_semantic/resources/mdtest/bidirectional.md

@@ -200,7 +200,7 @@ def f() -> list[Literal[1]]:
     return [1]
 ```
 
-## Instance attribute
+## Instance attributes
 
 ```toml
 [environment]
@@ -235,6 +235,24 @@ def _(flag: bool):
         C.x = lst(1)
 ```
 
+For union targets, each element of the union is considered as a separate type context:
+
+```py
+from typing import Literal
+
+class X:
+    x: list[int | str]
+
+class Y:
+    x: list[int | None]
+
+def lst[T](x: T) -> list[T]:
+    return [x]
+
+def _(xy: X | Y):
+    xy.x = lst(1)
+```
+
 ## Class constructor parameters
 
 ```toml