fix: flaky e2e test for dropping assets on nodes (#7352)

Fix flaky "Can drag-and-drop animated webp image" test that was reading
the widget value before the upload completed, causing intermittent
failures where filenames appeared truncated. Added `waitForUpload`
option to `dragAndDropFile` helper that waits for the `/upload/`
response before returning. This is opt-in since not all drag-and-drop
operations trigger uploads (e.g., loading workflows from media files).

browser_tests/tests/widget.spec.ts

@@ -252,7 +252,8 @@ test.describe('Animated image widget', () => {
 
     // Drag and drop image file onto the load animated webp node
     await comfyPage.dragAndDropFile('animated_webp.webp', {
-      dropPosition: { x, y }
+      dropPosition: { x, y },
+      waitForUpload: true
     })
 
     // Expect the filename combo value to be updated