Testing HTTP Calls with HttpTestingController

Testing HTTP services should not hit the real backend. Angular provides HttpTestingController to fake responses.

## Service

```ts
import { Injectable } from '@angular/core';
import { HttpClient } from '@angular/common/http';
import { Observable } from 'rxjs';

export type Product = { id: number; title: string };

@Injectable({ providedIn: 'root' })
export class ProductsApi {
  constructor(private http: HttpClient) {}

  getProducts(): Observable<Product[]> {
    return this.http.get<Product[]>('/api/products');
  }
}
```

## Test

```ts
import { TestBed } from '@angular/core/testing';
import { provideHttpClientTesting, HttpTestingController } from '@angular/common/http/testing';
import { provideHttpClient } from '@angular/common/http';
import { ProductsApi } from './products.api';

describe('ProductsApi', () => {
  it('returns products from API', () => {
    TestBed.configureTestingModule({
      providers: [ProductsApi, provideHttpClient(), provideHttpClientTesting()],
    });

    const api = TestBed.inject(ProductsApi);
    const httpMock = TestBed.inject(HttpTestingController);

    api.getProducts().subscribe(data => {
      expect(data.length).toBe(2);
      expect(data[0].title).toBe('Laptop');
    });

    const req = httpMock.expectOne('/api/products');
    expect(req.request.method).toBe('GET');

    req.flush([
      { id: 1, title: 'Laptop' },
      { id: 2, title: 'Phone' },
    ]);

    httpMock.verify();
  });
});
```

## What this gives you

- fast tests
- no real backend dependency
- ability to test error handling too

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