fix(vue-query): use environmentManager.isServer in plugin

[raashish1601]

Summary

• Use query core \environmentManager.isServer()\ in \VueQueryPlugin\ instead of importing \isServer\ directly.
• Add regression test ensuring \client.mount()\ is skipped when server mode is forced via \environmentManager.setIsServer().

Testing

• npx -y pnpm@11.1.0 --dir packages/vue-query test path: �itest via pnpm exec (command below)
  • 
    px -y pnpm@11.1.0 --dir C:\Users\raash\Desktop\open-source-contribution\tanstack-query-8639 --filter @tanstack/vue-query exec vitest run src/tests/vueQueryPlugin.test.ts --no-coverage\