fix(@angular/build): allow custom runner configuration file for unit-test

[clydin]

Adds a new runnerConfig option to the unit-test system to provide more control over the test runner's configuration file. This option is runner-agnostic and enhances flexibility for both Vitest and Karma.

The option accepts a boolean or a string path:

• true: Automatically searches for a default config file (karma.conf.js or vitest.config.ts). If not found, the runner will use its internal default configuration.
• false (default): Disables the use of any external config file.
• path/to/config: Uses the specified configuration file.

For Vitest, the loaded configuration is deep-merged with the system's programmatic config. This allows users to add or override advanced options (like test.coverage) while preserving the essential in-memory integration. The system's settings take precedence in case of conflicts.

Please note that while the file is loaded, the Angular team does not provide direct support for its specific contents or any third-party plugins used within it.

Informational messages are now logged to indicate which configuration file is being used, improving transparency.

Closes #31392

[yjaaidi]

Nice one @clydin, thanks!

Just one thing.
While I think that the feature is crucial and I know that a standalone vitest.config.ts requires more effort and is not the current goal, I am afraid that such a config file, especially when using the conventional name (vitest.config.ts) will actually confuse the integration with tools such as the IDE plugins as it’s a partial configuration.

To avoid user and tool confusion, what about naming it differently (e.g. vitest-overrides.config.ts or something)
Or what about exposing a defineVitestConfig function that strips the options that can't be overridden from the typing.
I can already imagine issues such as:

• I've changed this option in vitest.config.ts and when I run the tests using vitest, the options work but the tests involving Angular components do not work, and when I use the builder, the option seems to be ignored, but that other option is not ignored.

[clydin]

This is opt-in and the name can be configured to an arbitrary value. If there is need to avoid it being picked up by other tools (some actually may want it picked up), it can be named something different. Also of relevance, this is more of a "base" rather than an override. Options specified on command execution take precedence which follows the behavior of Vitest.

[alan-agius4]

Note: Not sure how to handle this, but having type both boolean or string will make runnerConfig config not possible to be used from the CLI. As options cannot be either boolean or string.

[yjaaidi]

Agreed, something like a base suffix would be a better fit.

I am actually concerned about the default name. Newcomers and AI-generated code will more likely pick vitest.config.ts and be surprised of the behavior.

[clydin]

Initial goal here was consistency as the Angular CLI has used the existing karma.conf.js default file name.
However, we can evaluate a different default but it would require implementing manual file discovery that would match vitest's.

[clydin]

This PR was merged into the repository. The changes were merged into the following branches:

• main: 20079ed
• 21.0.x: 542d528