distinguish references from definitions in infer_nonlocal

The initial implementation of `infer_nonlocal` landed in
https://github.com/astral-sh/ruff/pull/19112 fails to report an error
for this example:

```py
x = 1
def f():
    # This is only a usage of `x`, not a definition. It shouldn't be
    # enough to make the `nonlocal` statement below allowed.
    print(x)
    def g():
        nonlocal x
```

Fix this by continuing to walk enclosing scopes when the place we've
found isn't bound, declared, or `nonlocal`.

crates/ty_python_semantic/resources/mdtest/scopes/nonlocal.md

@@ -141,6 +141,12 @@ def f():
     global x
     def g():
         nonlocal x  # error: [invalid-syntax] "no binding for nonlocal `x` found"
+
+def f():
+    # A *use* of `x` in an enclosing scope isn't good enough. There needs to be a binding.
+    print(x)
+    def g():
+        nonlocal x  # error: [invalid-syntax] "no binding for nonlocal `x` found"
 ```
 
 A class-scoped `x` also doesn't work: