feat: Replace icon loading method with MenuUtils for Mac compatibility

Author: ethanyhou

com.microsoft.copilot.eclipse.ui/src/com/microsoft/copilot/eclipse/ui/handlers/ShowMenuBarMenuHandler.java

@@ -355,9 +355,8 @@ private CommandContributionItemParameter createCommandContributionItemParameter(
   }
 
   private void setDefaultBlankIcon(CommandContributionItemParameter parameter) {
-    ImageDescriptor icon = UiUtils.buildImageDescriptorFromPngPath("/icons/blank.png");
     if (PlatformUtils.isMac()) {
-      parameter.icon = icon;
+      parameter.icon = MenuUtils.getBlankIcon();
     }
   }
 }

com.microsoft.copilot.eclipse.ui/src/com/microsoft/copilot/eclipse/ui/handlers/ShowStatusBarMenuHandler.java

@@ -461,9 +461,8 @@ public static Action createMenuAction(MenuManager menuManager, String text, Stri
     }
 
     private static void setDefaultBlankIcon(Action action) {
-      ImageDescriptor blankIcon = UiUtils.buildImageDescriptorFromPngPath("/icons/blank.png");
       if (PlatformUtils.isMac()) {
-        action.setImageDescriptor(blankIcon);
+        action.setImageDescriptor(MenuUtils.getBlankIcon());
       }
     }
   }