[web] only emit assistant-add-on-purchase for add-on purchases (#30288)

GitOrigin-RevId: 084e28f02d348fe5504a38f1405250ea2f7b67ce

services/web/frontend/js/features/subscription/components/preview-subscription-change/root.tsx

@@ -69,9 +69,9 @@ function PreviewSubscriptionChange() {
         upgradeType: 'add-on',
         referrer: purchaseReferrer,
       })
+      eventTracking.sendMB('assistant-add-on-purchase')
     }
 
-    eventTracking.sendMB('assistant-add-on-purchase')
     payNowTask
       .runAsync(payNow(preview))
       .then(() => {