`TestBed.overrideComponent` breaks module imports with `@angular/build:unit-test`

### Command

test

### 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

Reopening from https://github.com/angular/angular/issues/68363 as I don't feel it was fully answered and I later found the issue is related to the new builder.

When using the `@angular/build:unit-test` builder, `TestBed.overrideComponent` breaks imports out of modules. This happens for both vitest and karma runners, so I would assume it is connected to the build, not the test execution.

The issue does not occur when using other builder like `@angular-devkit/build-angular:karma`

### Minimal Reproduction

Plain Angular project + packages needed for running karma

```typescript
// app.spec.ts
import { Component, NgModule } from '@angular/core';
import { TestBed } from '@angular/core/testing';

@Component({
selector: 'test-component',
template: '',
})
export class TestComponent {}

@NgModule({
imports: [TestComponent],
exports: [TestComponent],
})
export class TestModule {}

@Component({
selector: 'app-root',
imports: [TestModule],
template: ` `,
})
export class App {}

describe('App', () => {
beforeEach(async () => {
await TestBed.configureTestingModule({
imports: [App],
})
.overrideComponent(App, {}) // <= problematic call
.compileComponents();
});

it('should create', () => {
expect(TestBed.createComponent(App)).toBeTruthy();
});
});
```

```jsonc
// angular.json
...
// fails
"test-vitest": {
"builder": "@angular/build:unit-test",
"options": {
"runner": "vitest"
}
},
// fails
"test-karma": {
"builder": "@angular/build:unit-test",
"options": {
"runner": "karma"
}
},
// this one works
"test-karma-devkit": {
"builder": "@angular-devkit/build-angular:karma",
"options": {
"tsConfig": "./tsconfig.spec.json"
}
}
...
```

Running the test with `@angular/build:unit-test` it fails for both runners with
```
Error: NG0304: 'test-component' is not a known element (used in the '_App' component template):
1. If 'test-component' is an Angular component, then verify that it is included in the '@Component.imports' of this component.
2. If 'test-component' is a Web Component then add 'CUSTOM_ELEMENTS_SCHEMA' to the '@Component.schemas' of this component to suppress this message.
❯ validateElementIsKnown ../darwin_arm64-fastbuild-ST-fdfa778d11ba/bin/packages/core/src/render3/instructions/element_validation.ts:116:14
❯ validateElementIsKnown ../darwin_arm64-fastbuild-ST-fdfa778d11ba/bin/packages/core/src/render3/instructions/element.ts:151:4
❯ initializeElement ../darwin_arm64-fastbuild-ST-fdfa778d11ba/bin/packages/core/src/render3/instructions/element.ts:127:2
❯ ɵɵelementStart ../darwin_arm64-fastbuild-ST-fdfa778d11ba/bin/packages/core/src/render3/instructions/element.ts:218:2
❯ _App_Template ng:/_App.js:6:21
❯ templateFn ../darwin_arm64-fastbuild-ST-fdfa778d11ba/bin/packages/core/src/render3/instructions/shared.ts:109:4
❯ executeTemplate ../darwin_arm64-fastbuild-ST-fdfa778d11ba/bin/packages/core/src/render3/instructions/render.ts:107:6
❯ renderView ../darwin_arm64-fastbuild-ST-fdfa778d11ba/bin/packages/core/src/render3/instructions/render.ts:47:4
❯ renderComponent ../darwin_arm64-fastbuild-ST-fdfa778d11ba/bin/packages/core/src/render3/instructions/render.ts:160:4
❯ renderChildComponents ../darwin_arm64-fastbuild-ST-fdfa778d11ba/bin/packages/core/src/render3/instructions/render.ts:140:6
```

With the `@angular-devkit/build-angular:karma` it passes as expected.

Also actually overriding something in `TestBed.overrideComponent` doesn't change the behaviour. I originally discovered this with a test that needs to stub a component. I left it out for simplicity here.

### Exception or Error

```text

```

### Your Environment

```text
_ _ ____ _ ___
/ \ _ __ __ _ _ _| | __ _ _ __ / ___| | |_ _|
/ △ \ | '_ \ / _` | | | | |/ _` | '__| | | | | | |
/ ___ \| | | | (_| | |_| | | (_| | | | |___| |___ | |
/_/ \_\_| |_|\__, |\__,_|_|\__,_|_| \____|_____|___|
|___/

Angular CLI : 21.2.8
Angular : 21.2.10
Node.js : 24.15.0
Package Manager : npm 10.9.4
Operating System : win32 x64

┌───────────────────────────────┬───────────────────┬───────────────────┐
│ Package │ Installed Version │ Requested Version │
├───────────────────────────────┼───────────────────┼───────────────────┤
│ @angular-devkit/build-angular │ 21.2.8 │ ^21.2.8 │
│ @angular/build │ 21.2.8 │ ^21.2.7 │
│ @angular/cli │ 21.2.8 │ ^21.2.7 │
│ @angular/common │ 21.2.10 │ ^21.2.0 │
│ @angular/compiler │ 21.2.10 │ ^21.2.0 │
│ @angular/compiler-cli │ 21.2.10 │ ^21.2.0 │
│ @angular/core │ 21.2.10 │ ^21.2.0 │
│ @angular/forms │ 21.2.10 │ ^21.2.0 │
│ @angular/platform-browser │ 21.2.10 │ ^21.2.0 │
│ @angular/router │ 21.2.10 │ ^21.2.0 │
│ rxjs │ 7.8.2 │ ~7.8.0 │
│ typescript │ 5.9.3 │ ~5.9.2 │
│ vitest │ 4.1.5 │ ^4.0.8 │
└───────────────────────────────┴───────────────────┴───────────────────┘
```

### Anything else relevant?

_No response_

首先在 angular.json 中使用兩個 @angular/build:unit-test runner 設定執行 app.spec.ts 中的重現，然後將其與 @angular-devkit/build-angular:karma 進行比較。追蹤 TestBed.overrideComponent(App, {}) 如何影響匯入的 TestModule，並在兩個 runner 中使用提供的測試驗證修正；該測試應識別 test-component 並成功建立 App。

由索引模型根據 Issue 內容生成。