vp test fails with Yarn PnP: Cannot find package 'vite-plus'

## Description

`vp test` fails on projects using Yarn PnP (Plug'n'Play) mode. The bundled config file cannot resolve the `vite-plus` module because Yarn PnP does not use a traditional `node_modules` layout.

## Error

```
Error [ERR_MODULE_NOT_FOUND]: Cannot find package 'vite-plus' imported from vitest.config.mts.timestamp-xxx.mjs
```

The full error from `vp test run`:

```
vitest.config.mts (1:468) Warning: Could not resolve 'vite-plus' in vitest.config.mts

failed to load config from vitest.config.mts

⎯⎯⎯⎯⎯ Startup Error ⎯⎯⎯⎯⎯
Error [ERR_MODULE_NOT_FOUND]: Cannot find package 'vite-plus' imported from vitest.config.mts.timestamp-xxx.mjs
at Object.getPackageJSONURL (node:internal/modules/package_json_reader:301:9)
at packageResolve (node:internal/modules/esm/resolve:768:81)
...
```

## Reproduction

- Repository: https://github.com/fengmk2/vitest-extension-1.42.0-fail-yarn
- Branch: `vp-migrate-yarn-pnp`
- PR with CI failure: https://github.com/fengmk2/vitest-extension-1.42.0-fail-yarn/pull/1

Steps:
1. Clone the repo and checkout `vp-migrate-yarn-pnp`
2. Run `vp install`
3. Run `vp test run`

## Environment

- Vite+ version: `v0.0.0-g52709db6.20260226-1136`
- Yarn version: `4.12.0`
- Node.js: `24.14.0`
- OS: Ubuntu (GitHub Actions) / macOS

## Config

`vitest.config.mts`:
```ts
import { defineConfig } from 'vite-plus';
```

`package.json` (relevant parts):
```json
{
"packageManager": "yarn@4.12.0",
"devDependencies": {
"vite-plus": "catalog:"
},
"resolutions": {
"vite": "npm:@voidzero-dev/vite-plus-core@latest",
"vitest": "npm:@voidzero-dev/vite-plus-test@latest"
}
}
```

The project uses Yarn PnP (`.pnp.cjs` present, `nodeLinker` not set to `node-modules`).

## Expected Behavior

`vp test` should work with Yarn PnP projects, resolving `vite-plus` through Yarn's PnP resolution.