fix(metallb): verify the metallb-system namespace actually exists

- change the Test metallb-system namespace task to run
  k3s kubectl get namespace metallb-system instead of the bare
  -n metallb-system, which printed kubectl usage and always exited 0
- add a regression test that asserts the task uses an explicit get
  and fails if it ever regresses to the usage-only form
- wire the new test into pre-commit

roles/k3s_server_post/tasks/metallb.yml

@@ -40,7 +40,7 @@
 
 - name: Test metallb-system namespace
   ansible.builtin.command: >-
-    {{ k3s_kubectl_binary | default('k3s kubectl') }} -n metallb-system
+    {{ k3s_kubectl_binary | default('k3s kubectl') }} get namespace metallb-system
   changed_when: false
   with_items: "{{ groups[group_name_master | default('master')] }}"
   run_once: true