fix(@angular/build): disable Vitest test isolation by default

Disables test isolation in Vitest by setting `isolate: false` in the default
Vitest configuration. This change aligns the default test isolation behavior
with the existing Karma/Jasmine experience, promoting standardization across
testing frameworks within the Angular CLI.

This also provides significant performance improvements, especially in browser
mode, by reducing the overhead associated with test isolation. All known issues
related to disabling isolation have been addressed.

(cherry picked from commit 4d42a3e)

Author: clydin

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

@@ -117,7 +117,8 @@ export async function createVitestConfigPlugin(
         test: {
           setupFiles,
           globals: true,
-          // Allow Vitest to manage test isolation by its default behavior.
+          // Default to `false` to align with the Karma/Jasmine experience.
+          isolate: false,
           sequence: { setupFiles: 'list' },
         },
         optimizeDeps: {