fix: making import of ninja optional

[maticardenas]

As reported by @StopMotionCuber, after we introduced support for Django-Ninja's test client within the module clients.py, projects which doesn't have ninja installed began failing when attempting to import the former client.

This PR aims to fix this issue while defining an optional import and validation of the ninja library.

[StopMotionCuber]

Thanks for taking a look at my comment and working on it.

See my review comment, I think in the current form the fix does not work yet. Would be nice if you could take another look on it

[StopMotionCuber]

My test integration is still failing, but I guess for unrelated reasons. I'm using validate_response at some point and it seems that with 1.4.0 the expected request format has changed. Will do some deeper analysis tomorrow. This MR looks good

[StopMotionCuber]

With the fix of changing

schema_tester.validate_response(response)

to

schema_tester.validate_response(DRFResponseHandler(response))

my pipeline works again with your proposed patch. I mean it makes sense that it starts breaking if you change the API.

Here the question would be whether it makes sense to have a breaking API change or whether it would be a better idea to go with the previous code of

response_handler = ResponseHandlerFactory.create(response)

and integrate the detection "which type of response is it? DRF or Django Ninja?" there.

Anyways, I consider this comment to be out of scope for this PR and can confirm that the patch works.

[maticardenas]

@StopMotionCuber indeed validate_response's signature changed in version 1.4.0 in favor of using now the response handler (aiming for simplicity there). I agree the usage of the factory is the ideal in this case.

I will then merge this PR and trigger a release so that the issue with the absence of ninja is solved. Thanks for the help!