Use TUnit HTTP response assertions for API tests

[sliekens]

TUnit 1.14.0 introduced built-in assertions for HttpResponseMessage, so instead of writing:

Assert.That(response.StatusCode).IsEqualTo(HttpStatusCode.OK);

we can write:

await Assert.That(response).HasStatusCode(HttpStatusCode.OK);
// or with implicit success check:
await Assert.That(response).IsSuccessful();

This would reduce boilerplate across API tests that check response status codes and headers directly.

Introduced in TUnit v1.14.0.