[12.x] Add assertClientError method to TestResponse

[shane-zeng]

This PR adds the assertClientError to the TestResponse class.

It provides a convenient way to assert that the response status code is in the 4xx range, improving test readability when the specific client error code is not important.

This will allow to use:

$response->assertClientError();

instead of:

$this->assertTrue($response->isClientError());

This aligns with other expressive methods like assertServerError() and makes tests more readable and intention-revealing.