Revert bundling Vitest directly inside Vite+

### Description

Currently, there is no way to add an import subpath as a package override (example below)

```jsonc
{
"devDependencies": {
"vitest": "npm:vite-plus@0.2.8/test" // doesn't work!
}
}
```

This means that, despite supposedly being compatible with Vitest, packages and integrations may break now due to the absence of a `vitest` package to import from.

### Suggested solution

The old setup of overriding `vitest` with `@voidzero-dev/vite-plus-test` worked fine for this purpose. I propose that we bring it back, at least temporarily, if the Vite+ core team is dead set on bundling it. This way, integrations that check for the `vitest` package still work with Vite+.

### Alternative

Alternatives include migrating the entire ecosystem to use the `vite-plus/test` export which seems unlikely to happen (and also would break compatibility with vanilla Vitest unless packages have to check for both exports, which is another optional peer dependency required to be declared).

### Additional context

This isn't as much of a problem for Vite, tsdown, Rolldown, and Oxlint because their requirements are type-level imports or (in the case of Vite) is already bypassed by overriding the `vite` package. On the other hand, some Vitest reporters rely on extending existing reporters or programmatically running Vitest, which isn't covered in the current export model.

### Validations

- [x] Read the [Contributing Guidelines](https://github.com/voidzero-dev/vite-plus/blob/main/CONTRIBUTING.md).
- [x] Confirm this request is for Vite+ itself and not for Vite, Vitest, tsdown, Rolldown, or Oxc.
- [x] Check that there isn't already an issue requesting the same feature.

Start by tracing Vite+'s package export and dependency-override configuration, then compare it with the npm alias example and the former @voidzero-dev/vite-plus-test setup. Done means integrations that import or programmatically use the vitest package resolve under Vite+ without requiring migration to vite-plus/test.

Scritto dal modello di indicizzazione a partire dal testo della issue.