`unit-test` builder with `vitest` is slow for testing and for code coverage

### 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

I migrated a very small existing project using the jasmine-to-vitest schematic and code coverage is fairly slow when using vitest (roughly 80 tests).

With the `karma` builder:

```
ng test --no-watch 9,99s user 1,71s system 214% cpu 5,459 total
ng test --no-watch --code-coverage 17,82s user 2,42s system 232% cpu 8,719 total
```

When migrates to the unit-test builder (and a few tests disabled):

```
ng test --no-watch 18,49s user 3,33s system 259% cpu 8,396 total
ng test --no-watch --coverage 38,46s user 4,66s system 210% cpu 20,535 total
```

The tests themselves takes 18s (10s with Karma), and the code coverage takes 20s instead of 8s. This is very noticeable when using vitest.

### Minimal Reproduction

This is probably reproducible in a new CLI project, by generating a bunch of components and comparing test times.

### Exception or Error

```text

```

### Your Environment

```text
Angular CLI : 21.0.0-rc.1
Angular : 21.0.0-rc.1
Node.js : 22.18.0
Package Manager : npm 10.9.0
Operating System : darwin arm64

┌───────────────────────────┬───────────────────┬───────────────────┐
│ Package │ Installed Version │ Requested Version │
├───────────────────────────┼───────────────────┼───────────────────┤
│ @angular/build │ 21.0.0-rc.1 │ 21.0.0-rc.1 │
│ @angular/cli │ 21.0.0-rc.1 │ 21.0.0-rc.1 │
│ @angular/common │ 21.0.0-rc.1 │ 21.0.0-rc.1 │
│ @angular/compiler │ 21.0.0-rc.1 │ 21.0.0-rc.1 │
│ @angular/compiler-cli │ 21.0.0-rc.1 │ 21.0.0-rc.1 │
│ @angular/core │ 21.0.0-rc.1 │ 21.0.0-rc.1 │
│ @angular/forms │ 21.0.0-rc.1 │ 21.0.0-rc.1 │
│ @angular/localize │ 21.0.0-rc.1 │ 21.0.0-rc.1 │
│ @angular/platform-browser │ 21.0.0-rc.1 │ 21.0.0-rc.1 │
│ @angular/router │ 21.0.0-rc.1 │ 21.0.0-rc.1 │
│ rxjs │ 7.8.2 │ 7.8.2 │
│ typescript │ 5.9.3 │ 5.9.3 │
│ vitest │ 4.0.8 │ 4.0.8 │
```

### Anything else relevant?

Of course this is maybe an upstream problem, but I thought it was worth mentioning it.

まず、`unit-test` builder と Vitest を使用する小規模な Angular CLI プロジェクトで、`ng test --no-watch` コマンドと coverage コマンドを使って報告された時間を再現し、その後 Karma と比較します。移行に関係する unit-test builder と jasmine-to-vitest schematic のエントリーポイントを読みます。低速化の原因を特定し、テストと coverage の実行時間が改善されたことを示せれば完了です。

索引モデルが issue の本文から書いたものです。