Provide unit test capabilities

[cracksalad]

I guess a pretty common use case for this project is to create a proxy/adapter for some HTTP based API. This would for example look like this:

class OpenWeatherMapProxy {
  
  private readonly HttpClient $client;
  
  public function __construct(#[\SensitiveParameter] string $apiKey, string $lang = 'en') {
    $this->client = (new HttpClientBuilder())
        ->retry(3)
        ->intercept(new AddRequestHeader('Accept', 'application/json'))
        ->intercept(new ModifyRequest(function(Request $request) use($apiKey, $lang): Request {
          $request->setQueryParameter('appid', $apiKey);  // add API key to every request
          $request->setQueryParameter('units', 'metric'); // add preferred unit system to every request
          $request->setQueryParameter('lang', $lang);     // add preferred language to every request
          return $request;
        }))
        ->build();
  }

  // [...]
}

Now, how to unit or integration test this?
Note: we do not want to actually send requests, since the API might have a strict rate limit or is priced per API call. In addition, that would miss the point of unit and integration tests.

Approaches:

• Refactor the class to utilize Dependency Injection pattern: This would move the whole HttpClientBuilder part to the caller, which is weird, since we do some API specific things there. Or is there a way to extend a HttpClient with interceptors and stuff after its initial build? Maybe using new InterceptedHttpClient($paramClient, $myInterceptor1)?
• Insert a mocked HttpClient into the proxy and replace the $client using Reflection: This would make it impossible to test the HttpClient settings from the constructor and it is impossible due to readonly, which - apart from that - makes absolutely sense here.

Either way, in my opinion there should be a way to test stuff like this and it should be documented how to do it properly in the README.md. Thanks for helping me out here!