ng-add support for Vitest browser providers doesn't work

### Command

add

### 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

@angular/cli v21.2.0-next.0 added in https://github.com/angular/angular-cli/commit/f80db6fb714aa326f6ed03a8a51090ca59ad0955 `add ng-add support for Vitest browser providers` but it is not complete and does not actually work.

The schematic correctly adds the required packages and modifies `tsconfig.spec.json` but does not add the `browsers` option as documented at https://angular.dev/guide/testing/migrating-to-vitest#5-configure-browser-mode-optional.

Without this option the test doesn't actually run in browser environment and is still run in DOM emulation library within Node.js.

### Minimal Reproduction

Repro steps:

1. `ng new angular22dot2dot0next0 --ssr=false --style=scss --ai-config=none`
2. `ng add @vitest/browser-playwright`
3. `npx playwright install`
4. Open `app.spec.ts` and add line to any test: `console.log('RUNNING IN:', navigator.userAgent);`
5. Run test via `ng test`
6. Observe the console outputs: `RUNNING IN: Mozilla/5.0 (win32) AppleWebKit/537.36 (KHTML, like Gecko) jsdom/27.4.0`
7. Test still runs in Node.js with DOM emulation, not in Vitest browser ❌

How to fix:

1. Open `angular.json` and change:
```json
"test": {
"builder": "@angular/build:unit-test"
}
```

to:
```json
"test": {
"builder": "@angular/build:unit-test",
"options": {
"browsers": ["chromium"]
}
}
```
2. Run test via `ng test`
3. Observe actual Chrome for Testing window opens and Console outputs: `RUNNING IN: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/145.0.0.0 Safari/537.36`
4. Test runs in Vitest browser 👍

### Exception or Error

```text

```

### Your Environment

```text
_ _ ____ _ ___
/ \ _ __ __ _ _ _| | __ _ _ __ / ___| | |_ _|
/ △ \ | '_ \ / _` | | | | |/ _` | '__| | | | | | |
/ ___ \| | | | (_| | |_| | | (_| | | | |___| |___ | |
/_/ \_\_| |_|\__, |\__,_|_|\__,_|_| \____|_____|___|
|___/

Angular CLI : 21.2.0-next.0
Node.js : 24.13.0
Package Manager : npm 11.6.2
Operating System : win32 x64
```

### Anything else relevant?

_No response_

Reproduziere das Problem mit den aufgeführten Schritten für ng new, ng add, die Playwright-Installation und ng test. Vergleiche die erzeugten angular.json und tsconfig.spec.json mit der dokumentierten Konfiguration für den Browser-Modus und untersuche anschließend die durch den Commit f80db6f eingeführte ng-add-Unterstützung. Als abgeschlossen gilt die Aufgabe, wenn ng add die browsers-Option so konfiguriert, dass ng test in der Vitest-Browserumgebung statt in jsdom unter Node.js ausgeführt wird.

Vom Indexierungsmodell aus dem Issue-Text verfasst.