fix(@angular/build): prevent reporter duplicates by explicitly overriding Vitest configuration

When leveraging the Angular CLI to run tests with a specific set of `--reporters`, Vitest's underlying `mergeConfig` logic would normally array-concatenate the CLI reporters with any reporters defined natively inside `vitest.config.ts`. This led to duplicate output processors (e.g. running 'default' twice).

By explicitly wiping the original configurations when CLI overrides are present, the CLI now acts as a strict Source-of-Truth array replacer, which is the expected behavior for CI and custom targets.

Author: clydin

packages/angular/build/src/builders/unit-test/runners/vitest/plugins.ts

@@ -82,6 +82,18 @@ export async function createVitestConfigPlugin(
     async config(config) {
       const testConfig = config.test;
 
+      if (reporters !== undefined) {
+        delete testConfig?.reporters;
+      }
+
+      if (
+        options.coverage.reporters !== undefined &&
+        testConfig?.coverage &&
+        'reporter' in testConfig.coverage
+      ) {
+        delete testConfig.coverage.reporter;
+      }
+
       if (testConfig?.projects?.length) {
         this.warn(
           'The "test.projects" option in the Vitest configuration file is not supported. ' +

packages/angular/build/src/builders/unit-test/tests/behavior/runner-config-vitest_spec.ts

@@ -42,6 +42,21 @@ describeBuilder(execute, UNIT_TEST_BUILDER_INFO, (harness) => {
       harness.expectFile('vitest-results.xml').toExist();
     });
 
+    it('should override reporters defined in runnerConfig file when CLI option is present', async () => {
+      harness.useTarget('test', {
+        ...BASE_OPTIONS,
+        runnerConfig: 'vitest.config.ts',
+        reporters: ['default'],
+      });
+
+      harness.writeFile('vitest.config.ts', VITEST_CONFIG_CONTENT);
+
+      const { result } = await harness.executeOnce();
+      expect(result?.success).toBeTrue();
+      // The CLI option 'default' should override the 'junit' reporter in VITEST_CONFIG_CONTENT
+      harness.expectFile('vitest-results.xml').toNotExist();
+    });
+
     it('should use custom reportsDirectory defined in runnerConfig file', async () => {
       harness.useTarget('test', {
         ...BASE_OPTIONS,