feat(@angular/build): add isolate option to unit-test builder

[clydin]

This commit adds an isolate option to the @angular/build:unit-test builder for the Vitest runner. By default, tests run in a non-isolated environment to match the behavior of Karma/Jasmine. This new option allows developers to easily opt-in to Vitest native isolation (running tests in separate threads or processes) without requiring a custom Vitest configuration file. The option is not supported by the Karma runner and will result in an error if used there.

[gemini-code-assist]

Code Review

This pull request introduces an "isolate" option to the unit test builder, enabling test isolation when using the Vitest runner. The changes span the public API, schema definitions, option normalization, and the Vitest plugin configuration, supported by new unit tests. Review feedback identifies a bug in the validation logic where the default runner is not correctly accounted for, a conflict between isolation and debug modes, and a potential issue where CLI flags might be overridden by user configuration files.

[dgp1130]

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

• main: e9aa796

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