Acceptance-test helpers must build their own client

Trigger: A CheckDestroy or Check helper panics with interface conversion: interface {} is nil, not *clientset.ClientSet, often only when the test runs alone (-run '^TestX$') or in its own CI shard, such as a provider-migration job.

Fix: Build the client in the helper instead of reading it from testAccProvider.Meta():

cs, err := testAccNewClientSet()
if err != nil {
	return err
}

Why: testAccProvider is the SDKv2 provider, and it is only configured once some earlier test in the same process has used it. A framework-only or ExternalProviders test that runs alone never configures it, so Meta() is nil.