How to use a separate classes (stubs) in vitest?

I've migrated my angular 21 tests from jasmine to vitest. And for some of my tests I use separate helper classes (like Page object models). For example:

export class SignUpPageObject {
   constructor(fakeVerificationSvc: IUserVerificationService) {
      TestBed.overrideProvider(UserVerificationService, { useValue: fakeVerificationSvc });

      this.fixture = TestBed.createComponent(IlgSignUpForm);
      this.fixture.detectChanges();
      this.component = this.fixture.componentInstance;
      this.loader = TestbedHarnessEnvironment.loader(this.fixture);
      this.docRootLoader = TestbedHarnessEnvironment.documentRootLoader(this.fixture);
   }

   get emailInput(): Promise<MatInputHarness> {
      return this.loader.getHarness(MatInputHarness.with({ label: "Email" }));
   }

   async getErrorDialog(): Promise<MatDialogHarness> {
      const dlgHarnesses = await this.docRootLoader.getAllHarnesses(MatDialogHarness.with({ selector: "#ilgErrorDlg" }));
      return dlgHarnesses[0];
   }

   async getMessageDialog(): Promise<MatDialogHarness> {
      const dlgHarnesses = await this.docRootLoader.getAllHarnesses(MatDialogHarness.with({ selector: "#ilgMessageDlg" }));
      return dlgHarnesses[0];
   }

   async clickSendCodeBtn(): Promise<void> {
      const sendCodeBtn = await this.sendCodeBtn;
      await sendCodeBtn.click();
   }

   async isSendCodeBtnDisabled(): Promise<boolean> {
      return (await this.sendCodeBtn).isDisabled();
   }

   async emailHasErrors(): Promise<boolean> {
      const emailFld = await this.emailFld;
      return await emailFld.hasErrors();
   }

   async getEmailValue(): Promise<string> {
      return (await this.emailInput).getValue();
   }


   // ...


   async verificationCodeHasErrors(): Promise<boolean> {
      return (await this.verificationCodeFld).hasErrors();
   }

   async formErrorsCount(): Promise<number> {
      const errHarnesses = await this.loader.getAllHarnesses(MatErrorHarness);
      for (let errHarness of errHarnesses) {
         console.log(await errHarness.getText());
      }
      return errHarnesses.length;
   }

   async verificationCodeIsChecked(): Promise<boolean> {
      // there should <mat-icon>check</mat-icon> to appear
      return (await (await this.verifiedMatIcon).getName()) === "check";
   }

   readonly component: MyForm;
   readonly fixture: ComponentFixture<MyForm>;

   private readonly code?: number;
   private readonly loader: HarnessLoader;
   private readonly docRootLoader: HarnessLoader;
}

The name of this file is page-object.spec.ts. But when I run tests with vitest, I get an error:

No test suite found in file '...\page-object.spec.ts`

How should I name it so by glancing a name it would be clear that this file is part of the tests? Obviously, the *.spec.ts does not work.