fix: resolve fullscreen AX tree retrieval returning ref_count: 0

- Add !t.is_empty() check to prevent false positives in partial title matching
- Add fallback to select the first window with children > 0 using count_children API
- Ensures reliable active window selection in macOS native fullscreen mode

crates/macos/src/tree/builder.rs

@@ -31,11 +31,16 @@ pub fn window_element_for(pid: i32, win_title: &str) -> AXElement {
             let title = copy_string_attr(win, kAXTitleAttribute);
             if title
                 .as_deref()
-                .is_some_and(|t| t.contains(win_title) || win_title.contains(t))
+                .is_some_and(|t| !t.is_empty() && (t.contains(win_title) || win_title.contains(t)))
             {
                 return win.clone();
             }
         }
+        for win in &windows {
+            if count_children(win, None) > 0 {
+                return win.clone();
+            }
+        }
         if let Some(first) = windows.into_iter().next() {
             return first;
         }