[ty] fix lookup order of class variables before they are defined (#19743)

## Summary

This is a follow-up to #19321.

If we try to access a class variable before it is defined, the variable
is looked up in the global scope, rather than in any enclosing scopes.

Closes https://github.com/astral-sh/ty/issues/875.

## Test Plan

New tests in `narrow/conditionals/nested.md`.

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

@@ -346,7 +346,10 @@ l: list[str | Literal[1] | None] = [None]
 
 def f(x: str | Literal[1] | None):
     class C:
-        if x is not None:  # TODO: should be an unresolved-reference error
+        # If we try to access a variable in a class before it has been defined,
+        # the lookup will fall back to global.
+        # error: [unresolved-reference]
+        if x is not None:
             def _():
                 if x != 1:
                     reveal_type(x)  # revealed: str | None