`convert_to_function_declaration` fails for top level variables

Similar to #61815 

Example:

```dart
final v1 = () => 42;

void f() {
  v1();
}
```

should be fixed to:

```dart
int v1() => 42;

void f() {
  v1();
}
```

Dart SDK version: 3.11.0-edge.945f32dfc76fd029f8a0e782107b4fc6aaefa924 (main) (Fri Oct 24 06:15:16 2025 -0700) on "macos_arm64"