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_

Start by reproducing the listed Angular 21 browser-mode unit-test setup with Vitest and an import from `msw`; then inspect how the Angular unit test runner passes resolve conditions. Done means the browser-mode tests can import `msw` without the entry-point externalization error, while the jsdom behavior remains unchanged.

Written by the indexing model from the issue text.