Unhandled promise rejections aren't properly reported in tests

Reproduce the issue with the provided spec in a new Angular project and run npm t, comparing zoneless and zone.js configurations. The relevant project files are not named; use the test setup and the minimal reproduction at https://github.com/sgravrock/ng-unhandled-rejection as entry points. Done means the spec reports exactly one unhandled rejection in both configurations.

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

Don't known / other

Is this a regression?

Yes

Description

In an Angular project that uses zone.js, unhandled promise rejections are not reported. In a zoneless Angular project, they are reported twice.

Steps to reproduce:

1. Create a new Angular project.
2. Paste the code below into a spec file.
3. npm t

it('triggers an unhandled rejection', function(done) {
	Promise.reject(new Error('nope'));

	// Make sure the unhandled rejection event has plenty of time to fire before
	// Jasmine finishes running. This helps distinguish between timing issues
	// (event is firing a bit later than usual) and the event not firing at all.
	setTimeout(done, 1000);
});

Expected result: The spec should fail with a single "Unhandled promise
rejection: Error: nope" error.

Observed:

• If the project was set up zoneless, the spec fails with two copies of the expected error.
• Otherwise, the spec passes.

Please provide a link to a minimal reproduction of the bug

https://github.com/sgravrock/ng-unhandled-rejection

Please provide the exception or error you saw

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

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

Angular CLI: 20.1.5
Node: 20.19.0
Package Manager: npm 10.8.2
OS: darwin arm64

Angular: 20.1.6
... common, compiler, compiler-cli, core, forms
... platform-browser, router

Package                      Version
------------------------------------------------------
@angular-devkit/architect    0.2001.5
@angular-devkit/core         20.1.5
@angular-devkit/schematics   20.1.5
@angular/build               20.1.5
@angular/cli                 20.1.5
@schematics/angular          20.1.5
rxjs                         7.8.2
typescript                   5.8.3
zone.js                      0.15.1

Anything else?

No response