refactor(@angular/build): deprecate splitting option in unit-test builder

DEPRECATED: The `splitting` option in the unit-test builder is deprecated as it is no longer needed with Vitest 5.

Disabling code splitting is no longer necessary with Vitest 5, as the issue where shared-module exports were uninitialized in class-field initializers under JSDOM has been resolved.

Author: alan-agius4

packages/angular/build/src/builders/unit-test/schema.json

@@ -21,7 +21,7 @@
     },
     "runnerConfig": {
       "type": ["string", "boolean"],
-      "description": "Specifies the configuration file for the selected test runner. If a string is provided, it will be used as the path to the configuration file. If `true`, the builder will search for a default configuration file (e.g., `vitest-base.config.ts` or `karma.conf.js`). If `false`, no external configuration file will be used.\\nFor Vitest, this enables advanced options and the use of custom plugins. 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.",
+      "description": "Specifies the configuration file for the selected test runner. If a string is provided, it will be used as the path to the configuration file. If `true`, the builder will search for a default configuration file (e.g., `vitest-base.config.mts` or `karma.conf.js`). If `false`, no external configuration file will be used.\\nFor Vitest, this enables advanced options and the use of custom plugins. 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.",
       "default": false
     },
     "browsers": {
@@ -80,7 +80,8 @@
     "splitting": {
       "type": "boolean",
       "description": "Enables code splitting for test execution. When enabled, shared code between test files is split into separate chunks. Issues with live ESM bindings in Node.js environments (such as uninitialized exports or mocking failures) can be resolved by disabling splitting. This option is only available for the Vitest runner.",
-      "default": true
+      "default": true,
+      "x-deprecated": "No longer needed with Vitest 5."
     },
     "quiet": {
       "type": "boolean",

packages/angular/build/src/builders/unit-test/tests/behavior/vitest-shared-chunk-init_spec.ts

@@ -16,28 +16,11 @@ import {
 
 describeBuilder(execute, UNIT_TEST_BUILDER_INFO, (harness) => {
   describe('Behavior: "Vitest shared chunk initialization"', () => {
-    // Regression test for https://github.com/angular/angular-cli/issues/33728.
-    //
-    // Without `disableCodeSplitting`, esbuild hoists a module imported by more than one spec
-    // entry point into a shared chunk behind a lazy `__esm` initializer, and a class-field
-    // initializer in another chunk reads the exported value as `undefined` under the jsdom
-    // runner. All four trigger conditions are required and encoded below:
-    //   1. two spec entry points import the shared module (so it lands in a shared chunk);
-    //   2. a component in one entry reads the export during class-field initialization;
-    //   3. that component's spec file contains an `async` test callback (no `await` needed);
-    //   4. zone.js is in the polyfills (the `setupApplicationTarget` default), which downlevels
-    //      async and makes esbuild emit the spec entry CommonJS-wrapped.
-    //
-    // NOTE: the failure this guards against is sensitive to inert content — adding a top-level
-    // side effect (even a `console.log`) to the shared or importing module below defused it
-    // during reduction. Mirror https://github.com/jonmarozick/ng-shared-chunk-repro when
-    // modifying these fixtures.
     it('should provide shared-module exports to class-field initializers in async specs', async () => {
       setupApplicationTarget(harness);
 
       harness.useTarget('test', {
         ...BASE_OPTIONS,
-        splitting: false,
       });
 
       // Keep the default project's spec deterministic; a third spec entry that does not touch