Fix vitest version mismatch - upgrade vitest to 4.0.18

Co-authored-by: hotlong <50353452+hotlong@users.noreply.github.com>

Author: Copilot

packages/spec/package.json

@@ -93,7 +93,7 @@
     "@vitest/coverage-v8": "^4.0.18",
     "tsx": "^4.21.0",
     "typescript": "^5.3.0",
-    "vitest": "^2.1.8",
+    "vitest": "^4.0.18",
     "zod-to-json-schema": "^3.25.1"
   },
   "dependencies": {