fix(docs-infra): fix StackBlitz and zipped http examples (#33941)

Previously, the generated StackBlitz examples as well as the
corresponding downloadable zips for the `http` guide examples were not
correct and thus trying to run the app and/or tests would fail.

This commit fixes the examples:

- Replace `TestBed.inject()` (which was [introduced in v9][1]) with
  `TestBed.get()` (which is available in v8 used in the examples).
  (NOTE: The examples will soon be updated to v9 (as part of
  [FW-1609][2] and switched back to `TestBed.inject()` then.)

- Include `src/app/heroes/hero.ts` in the zip, because it is referenced
  by some of the other files and the compilation fails without it.

- Ensure `src/main-specs.ts` is not included in the zip that does not
  include the tests. Including the file broke the app, because there is
  logic in our zip-builder that renamed `main-*.ts` files to `main.ts`
  and thus `main-specs.ts` ended up overwriting the actual `main.ts`.

[1]: https://next.angular.io/guide/deprecations#angularcoretesting
[2]: https://angular-team.atlassian.net/browse/FW-1609

Fixes #33874
Fixes #33945

PR Close #33941

aio/content/examples/http/src/main.ts

@@ -1,5 +1,12 @@
 // #docregion
+import { enableProdMode } from '@angular/core';
 import { platformBrowserDynamic } from '@angular/platform-browser-dynamic';
+
 import { AppModule } from './app/app.module';
+import { environment } from './environments/environment';
+
+if (environment.production) {
+  enableProdMode();
+}
 
 platformBrowserDynamic().bootstrapModule(AppModule);