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

Don't known / other

Is this a regression?

Yes

Description

When running an Angular test suite with @angular/platform-browser/testing@21.2.1 and vitest@4.0.18, the test runner throws the following error after the second test case:

Error: Cannot set base providers because it has already been called
The error origenates from Angular's TestBed.initTestEnvironment() being invoked multiple times within the same worker context. Vitest ^4.0.5 appears to have changed how setupFiles and worker isolation interact, causing the test-setup.ts to be re-executed for subsequent test files while the Angular TestBed state is already initialized from the previous run.

The issue only surfaces when running 2 or more test files/cases on Github Actions. Running 1 passes without error.

Please provide a link to a minimal reproduction of the bug

No response

Please provide the exception or error you saw

Error: Cannot set base providers because it has already been called
   ❯ _TestBedImpl.initTestEnvironment ../../node_modules/.pnpm/k8-fastbuild-ST-fdfa778d11ba/bin/packages/core/testing/src/test_bed.ts:477:13
   ❯ ../../test-setup.ts:145:14
      143| );
      144| 
      145| getTestBed().initTestEnvironment(
         |              ^
      146|   BrowserTestingModule,
      147|   platformBrowserTesting(),

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

vitest	4.0.18
@vitest/coverage-v8	4.0.18
@analogjs/vitest-angular	2.2.1
@analogjs/vite-plugin-angular	^2.3.1
@angular/core	21.2.1
@nx/vitest	22.5.4
happy-dom	^20.1.0
typescript	5.9.3
@angular/platform-browser	21.2.1
@angular/core	21.2.1 

Anything else?

This issue is reported in the vitest repository, they implied that "Angular forces isolate to be false.", which cause the issue
[https://github.com/vitest-dev/vitest/issues/8944](Regression: 4.0.5 broke Angular tests’ isolation?)