MSW fails to import when running unit tests in browser mode

### Which @angular/* package(s) are the source of the bug?

compiler

### Is this a regression?

Yes

### Description

In Angular 21, when running unit tests using the below configuration the run fails with "The entry point "msw" cannot be marked as external"

Setup:
1. Create a new angular project.
2. Set unit tests to use the chromium browser
3. Install MSW
4. Attempt to import anything from `msw` in a test case, the above error is thrown.

This appears to be related to a disconnect between the `package.json` configuration of MSW and the "resolve conditions" that are being passed to Vitest by the angular unit test runner.

When running the tests in jsdom (not browser) we do not encounter this issue.

### Please provide a link to a minimal reproduction of the bug

_No response_

### Please provide the exception or error you saw

```true

```

### Please provide the environment you discovered this bug in (run `ng version`)

```true
Angular CLI : 21.1.4
Angular : 21.1.5
Node.js : 22.18.0
Package Manager : npm 11.5.2
Operating System : win32 x64

┌───────────────────────────┬───────────────────┬───────────────────┐
│ Package │ Installed Version │ Requested Version │
├───────────────────────────┼───────────────────┼───────────────────┤
│ @angular/build │ 21.1.4 │ ^21.1.4 │
│ @angular/cli │ 21.1.4 │ ^21.1.4 │
│ @angular/common │ 21.1.5 │ ^21.1.0 │
│ @angular/compiler │ 21.1.5 │ ^21.1.0 │
│ @angular/compiler-cli │ 21.1.5 │ ^21.1.0 │
│ @angular/core │ 21.1.5 │ ^21.1.0 │
│ @angular/forms │ 21.1.5 │ ^21.1.0 │
│ @angular/platform-browser │ 21.1.5 │ ^21.1.0 │
│ @angular/router │ 21.1.5 │ ^21.1.0 │
│ rxjs │ 7.8.2 │ ~7.8.0 │
│ typescript │ 5.9.3 │ ~5.9.2 │
│ vitest │ 4.0.18 │ ^4.0.8 │
└───────────────────────────┴───────────────────┴───────────────────┘
```

### Anything else?

_No response_

先使用 Vitest 以及從 `msw` 的匯入，重現列出的 Angular 21 瀏覽器模式單元測試設定；接著檢查 Angular 單元測試執行器如何傳遞 resolve conditions。完成的標準是：瀏覽器模式測試可以匯入 `msw`，且不會出現進入點外部化錯誤，同時 jsdom 的行為維持不變。

由索引模型根據 Issue 內容生成。