fix(tasks): include verify_vars.yaml as tasks file

The variable verification was included via include_vars, which is meant for variable files only. Additionally the
first_found lookup had no paths defined, so it searched the files/ subdirectory and the role root instead of tasks/ and
failed with "No file was found". Switching to include_tasks and adding the tasks path makes the verification run at all.

The assertion itself was fully commented out and is now active, so a misconfigured entry fails early instead of being
silently skipped by all four sudoers tasks.

Co-authored-by: Copilot <copilot@github.com>

tasks/verify_vars.yaml

@@ -1,8 +1,12 @@
 ---
 
-# - name: Verify if not user and group exists for each entry
-#   ansible.builtin.assert:
-#     that:
-#     - (item.user is defined and item.group is not defined) or
-#       (item.user is not defined and item.group is defined)
-#   with_items: "{{ sudo_users_sudoers }}"
+- name: Verify that each entry defines either a user or a group
+  ansible.builtin.assert:
+    that:
+    - (item.user is defined and item.user | length > 0) !=
+      (item.group is defined and item.group | length > 0)
+    fail_msg: >
+      Each entry of sudo_users_sudoers must define either 'user' or 'group',
+      but not both and not none.
+    quiet: true
+  with_items: "{{ sudo_users_sudoers }}"