Angular 21 new unit test builder with Karma and zone.js

### Command

test

### Is this a regression?

- [x] Yes, this behavior used to work in the previous version

### The previous version in which this bug was not present was

20

### Description

After upgrading from Angular 20 to 21, I ran into NGXXXX errors when running unit tests still with Karma and zone.js.
I found a [closed issue](https://github.com/angular/angular-cli/issues/32047) and a supposed fix for it, but that did not resolve the issue.
Others experienced the same and implemented workarounds which actually do work: [StackOverflow](https://stackoverflow.com/a/79837997), [Reddit](https://www.reddit.com/r/angular/comments/1p8suok/upgrading_from_angular_20_21_app_works_but_tests/).

I found that the solution could be that the `angular.json` has to be modified to be in line with the new project template like this:
- change `"builder": "@angular/build:karma`" to `"builder": "@angular/build:unit-test"`
- add `"runner": "karma"`
- remove the rest of the properties (according to [documentation](https://angular.dev/guide/testing/migrating-to-vitest#:~:text=does%20not%20support%20this) these derive from the development configuration)

I did not find anything regarding the necessity of this change in the documentation, but if that is the case, it would be wise to mention it and maybe even add a migration schematic.

Also its not clear what is the alternative for `"main": "src/test.ts"` which contains global test setup. My guess is that the `"setupFiles"` property would be the successor (like `"karmaConfig"` became `"runnerConfig"`), but that is [disabled for Karma](https://github.com/angular/angular-cli/blob/v21.2.5/packages/angular/build/src/builders/unit-test/runners/karma/executor.ts#L40).

So my question is: Where should I put global test setup with the new `"unit-test"` builder when using Karma?

Thanks in advance.

### Minimal Reproduction

npm test

### Exception or Error

```text

```

### Your Environment

```text
Angular CLI : 21.2.5
Node.js : 24.14.0
Package Manager : npm 11.9.0
Operating System : win32 x64
```

### Anything else relevant?

_No response_

Beginne mit packages/angular/build/src/builders/unit-test/runners/karma/executor.ts und vergleiche die Konfiguration des Unit-Test-Builders für Angular 20 und 21 in angular.json. Lies die im Report verlinkte Dokumentation zur Testmigration und verfolge, wie Karma das globale Test-Setup handhabt. Als erledigt gilt die Aufgabe, wenn die erwartete Karma-Konfiguration eingerichtet ist und die Dokumentation oder das Migrationsverhalten den gemeldeten Upgrade-Pfad abdeckt.

Vom Indexierungsmodell aus dem Issue-Text verfasst.