`@angular/build:unit-test` with Vitest test runner throws `Failed to import test file` error

### Command

test

### Is this a regression?

- [ ] Yes, this behavior used to work in the previous version

### The previous version in which this bug was not present was

_No response_

### Description

Steps to reproduce:

1. Create a `TestService` that will contain a single property `router = inject(Router)`
2. Configure `@angular/build:unit-test` runner to include `providersFile`
3. Export `[TestService]` from it
4. Create a `Test` component that will inject this service: `service = inject(TestService)`

`ng test` will fail with the following error **for all specs**:

```
FAIL project (chromium) path/to/some.spec.ts [ path/to/some.spec.ts ]
Error: Failed to import test file /project/root/init-testbed.js
Caused by: Error: The injectable '_PlatformLocation' needs to be compiled using the JIT compiler, but '@angular/compiler' is not available.

The injectable is part of a library that has been partially compiled.
However, the Angular Linker has not processed the library such that JIT compilation is used as fallback.

Ideally, the library is processed using the Angular Linker to become fully AOT compiled.
Alternatively, the JIT compiler should be loaded by bootstrapping using '@angular/platform-browser-dynamic' or '@angular/platform-server',
or manually provide the compiler with 'import "@angular/compiler";' before bootstrapping.
❯ node_modules/.vite/vitest/dc540563f7f56f2cac74b38788615874d1da1c10/deps/chunk-42LDSDVL.js?v=049c4943:83:17
```

If you remove injection of the `TestService` from the `Test` component **all the specs** will start to pass.

### Minimal Reproduction

https://github.com/th0r/ng-vitest-test-providers-bug

### Your Environment

```
Angular CLI : 21.0.1
Angular : 21.0.2
Node.js : 24.8.0
Package Manager : npm 11.6.0
Operating System : darwin arm64

┌───────────────────────────┬───────────────────┬───────────────────┐
│ Package │ Installed Version │ Requested Version │
├───────────────────────────┼───────────────────┼───────────────────┤
│ @angular/build │ 21.0.1 │ ^21.0.1 │
│ @angular/cli │ 21.0.1 │ ^21.0.1 │
│ @angular/common │ 21.0.2 │ ^21.0.0 │
│ @angular/compiler │ 21.0.2 │ ^21.0.0 │
│ @angular/compiler-cli │ 21.0.2 │ ^21.0.0 │
│ @angular/core │ 21.0.2 │ ^21.0.0 │
│ @angular/forms │ 21.0.2 │ ^21.0.0 │
│ @angular/platform-browser │ 21.0.2 │ ^21.0.0 │
│ @angular/router │ 21.0.2 │ ^21.0.0 │
│ rxjs │ 7.8.2 │ ~7.8.0 │
│ typescript │ 5.9.3 │ ~5.9.2 │
│ vitest │ 4.0.14 │ ^4.0.8 │
└───────────────────────────┴───────────────────┴───────────────────┘
```

Start with the @angular/build:unit-test runner's providersFile handling and the generated init-testbed.js, then reproduce the failure using the linked minimal repository. Verify the fix by running ng test with TestService injecting Router and confirming that all specs import and pass without the JIT compiler error.

Written by the indexing model from the issue text.