[12.x] Add assertRedirectBackWithErrors to TestResponse

[AhmedAlaa4611]

Description

This PR introduces a new assertRedirectBackWithErrors() method to streamline testing redirect-back responses that include validation errors.

Currently, developers need to write:

$response->assertRedirectBack();
$response->assertSessionHasErrors([
    // ...
]);

With this addition, the test can be simplified to:

$response->assertRedirectBackWithErrors([
    // ...
]);

Internally, this method simply calls assertRedirectBack() and assertSessionHasErrors(), so it's a convenience wrapper that improves test readability and reduces duplication.

About tests

I didn't include a separate test for this method since it delegates entirely to two existing, well-tested assertions.

[taylorotwell]

It raises more permutations in my mind... should there also be assertRedirectBackWithoutErrors?

[AhmedAlaa4611]

@taylorotwell

It raises more permutations in my mind... should there also be assertRedirectBackWithoutErrors?

That's a good point; I hadn't considered that, but I agree it makes sense for completeness and symmetry. I'm happy to add assertRedirectBackWithoutErrors() as well.

Should I make it in the same PR or in another PR?

[taylorotwell]

I got it! Thanks.