Switch from using the key window to the main window mac platform API

When the help menu is open, the help menu's search field is the key window, and this was causing menu item action resolution to fail

co-authored-by: Max <max@zed.dev>

crates/gpui/src/platform/test.rs

@@ -157,7 +157,7 @@ impl super::Platform for Platform {
         }))
     }
 
-    fn key_window_id(&self) -> Option<usize> {
+    fn main_window_id(&self) -> Option<usize> {
         None
     }