fix: Add missing vitest coverage dependency

CI was failing because test:coverage needs @vitest/coverage-v8

Author: jonmartin721

DocumentGenerator.Client/package-lock.json

@@ -31,6 +31,7 @@
     "@types/react": "^19.2.2",
     "@types/react-dom": "^19.2.2",
     "@vitejs/plugin-react": "^5.1.1",
+    "@vitest/coverage-v8": "^4.0.14",
     "eslint": "^9.39.1",
     "eslint-plugin-react-hooks": "^7.0.1",
     "eslint-plugin-react-refresh": "^0.4.24",