fix: add zod devDependency to Vercel provider (peer dep of ai v5)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: jsonbailey

packages/ai-providers/server-ai-vercel/package.json

@@ -53,6 +53,7 @@
     "@typescript-eslint/parser": "^6.20.0",
     "ai": "^5.0.0",
     "eslint": "^8.45.0",
+    "zod": "^3.25.76",
     "eslint-plugin-import": "^2.27.5",
     "eslint-plugin-jest": "^27.6.3",
     "jest": "^29.6.1",