[ty] Use declared variable types as bidirectional type context (#20796)

## Summary

Use the declared type of variables as type context for the RHS of assignment expressions, e.g.,
```py
x: list[int | str]
x = [1]
reveal_type(x)  # revealed: list[int | str]
```

crates/ty_python_semantic/resources/mdtest/narrow/conditionals/nested.md

@@ -310,13 +310,13 @@ no longer valid in the inner lazy scope.
 def f(l: list[str | None]):
     if l[0] is not None:
         def _():
-            reveal_type(l[0])  # revealed: str | None | Unknown
+            reveal_type(l[0])  # revealed: str | None
         l = [None]
 
 def f(l: list[str | None]):
     l[0] = "a"
     def _():
-        reveal_type(l[0])  # revealed: str | None | Unknown
+        reveal_type(l[0])  # revealed: str | None
     l = [None]
 
 def f(l: list[str | None]):