fix: use .first() in waitForNodes to avoid strict mode violation

waitForSelector tolerates multiple matches but locator().waitFor()
enforces strict single-element matching. The default workflow has
7 nodes, so the bare locator always fails.

Amp-Thread-ID: https://ampcode.com/threads/T-019d797f-578f-7399-bc07-e278fa246f3f
Co-authored-by: Amp <amp@ampcode.com>

browser_tests/fixtures/VueNodeHelpers.ts

@@ -146,7 +146,7 @@ export class VueNodeHelpers {
         expectedCount
       )
     } else {
-      await this.page.locator('[data-node-id]').waitFor()
+      await this.page.locator('[data-node-id]').first().waitFor()
     }
   }