chore: bump @percy/cli to ^1.31.15-beta.0 in tests (PER-7348)

CLI 1.31.15-beta.0 ships PercyDOM.waitForReady (the readiness gate). The SDK changes in this PR call waitForReady end-to-end in tests; old CLI pins (1.30.9, 1.31.10) don't have it, causing the typeof guard's done() callback path to never quite settle in geckodriver's async-script handling. Bump so tests run against a CLI that actually has the feature.

Author: Shivanshu-07

package.json

@@ -4,6 +4,6 @@
     "test": "npx percy exec --testing -- mvn test"
   },
   "devDependencies": {
-    "@percy/cli": "1.31.10"
+    "@percy/cli": "^1.31.15-beta.0"
   }
 }