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_

Tái hiện vấn đề bằng các bước ng new, ng add, cài đặt Playwright và ng test được liệt kê. So sánh angular.json và tsconfig.spec.json được tạo ra với cấu hình chế độ trình duyệt được ghi trong tài liệu, sau đó kiểm tra phần hỗ trợ ng-add được giới thiệu bởi commit f80db6f. Công việc được xem là hoàn tất khi ng add cấu hình tùy chọn browsers để ng test chạy trong môi trường trình duyệt của Vitest thay vì jsdom trong Node.js.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.