fix(@angular/build): disable root config extension for inline test project

In Vitest 5, inline projects declared under `test.projects` extend the
declaring root configuration by default (`extends: true`). Because the
Angular CLI test runner already explicitly merges user-provided options
from the configuration file into `projectConfig`, automatic root inheritance
caused array options—such as `browser.instances` and plugins—to be merged
and concatenated twice.

Setting `extends: false` on the inline project configuration keeps it
isolated from the declaring root config and prevents duplicate browser
instance registrations and repeated test runs.

Author: alan-agius4

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

@@ -245,6 +245,7 @@ export async function createVitestConfigPlugin(
       const projectResolver = createProjectResolver(projectSourceRoot);
 
       const projectDefaults: Vite.UserConfig & UserWorkspaceConfig = {
+        extends: false,
         test: {
           setupFiles,
           globals: true,
@@ -265,6 +266,7 @@ export async function createVitestConfigPlugin(
 
       const { optimizeDeps, resolve } = config;
       const projectOverrides: Vite.UserConfig & UserWorkspaceConfig = {
+        extends: false,
         test: {
           name: projectName,
           include,