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/testing/http-client.spec.ts

@@ -27,8 +27,8 @@ describe('HttpClient testing', () => {
     });
 
     // Inject the http service and test controller for each test
-    httpClient = TestBed.inject(HttpClient);
-    httpTestingController = TestBed.inject(HttpTestingController);
+    httpClient = TestBed.get(HttpClient);
+    httpTestingController = TestBed.get(HttpTestingController);
   });
   // #enddocregion setup
   // #docregion afterEach
@@ -67,6 +67,7 @@ describe('HttpClient testing', () => {
     httpTestingController.verify();
   });
   // #enddocregion get-test
+
   it('can test HttpClient.get with matching header', () => {
     const testData: Data = {name: 'Test Data'};