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_

Inizia riproducendo la configurazione indicata dei test unitari di Angular 21 in modalità browser con Vitest e un import da `msw`; quindi esamina come il runner dei test unitari di Angular passa le condizioni di risoluzione. Il lavoro è completato quando i test in modalità browser possono importare `msw` senza l’errore di esternalizzazione dell’entry point, mentre il comportamento di jsdom rimane invariato.

Scritto dal modello di indicizzazione a partire dal testo della issue.