fix(@angular/build): re-enable code splitting for unit tests

Re-enables esbuild code splitting for unit test builds by removing
`disableCodeSplitting` and resolving the underlying live export binding
issue when chunks are loaded by Vitest under Zone.js (#33728):

1. Ensures test spec files are treated as ES modules by appending
   `export {};` in the Angular compiler plugin if absent. In Zone.js
   applications, downleveled `async` functions capture module-level `this`
   via `__async(this, ...)`. Without an explicit export, esbuild misclassified
   spec files as CommonJS and wrapped them in `__commonJS`, which led to lazy
   `__esm` initializers for shared dependencies.
2. In the Vitest in-memory provider plugin, eagerly invokes any lazy `__esm`
   initializers detected in chunks when loaded for `vite-node`. Because
   `vite-node` statically captures exports upon initial chunk evaluation,
   eagerly executing the initializers ensures exported values are populated
   before consumers read them.
3. Enforces `namedChunks: false` for unit test builds to ensure chunks
   consistently match chunk naming patterns.

closes #33948

Author: alan-agius4

packages/angular/build/src/tools/esbuild/angular/compiler-plugin.ts

@@ -884,5 +884,7 @@ const SPECS_REGEXP = /\.(?:spec|test)\.[cm]?[jt]sx?$/;
  * (https://github.com/angular/angular-cli/issues/33728). Adding an empty export guarantees the file is bundled as an ES module.
  */
 function ensureTestFileEsm(request: string, contents: string): string {
-  return SPECS_REGEXP.test(request) ? contents + '\nexport {};\n' : contents;
+  return SPECS_REGEXP.test(request) && !contents.includes('export {};')
+    ? contents + '\nexport {};\n'
+    : contents;
 }