fix(ansible): the NVMe node would have died on an undefined variable

Self-review of the role I had just written. On k3s-prod-2 the scheduler path is
deliberately skipped, so the `stat` on /sys/fs/cgroup/system.slice/k3s.service is
skipped too — and a SKIPPED registration has no `.stat` key at all. Ansible
templates `loop:` BEFORE it evaluates `when:`, so

    loop: "{{ ['system.slice'] + (['system.slice/k3s.service']
              if _eio_k3s_cgroup.stat.exists else []) }}"

raises on that host even though the task would never have run. The role would have
failed on the one node it is designed to leave alone — and it would have failed at
the very end, after doing nothing, which reads like a broken mitigation rather than
an intentional skip.

`| default(false)` on both uses. Exercised offline against a simulated skipped
registration; `--syntax-check` re-run.

Co-Authored-By: Claude Opus 5 <[email protected]>
(cherry picked from commit 5ccf5111a54857abb934a64c41332b7aa5046323)

infra/ansible/roles/etcd_io_priority/tasks/main.yml

@@ -197,7 +197,7 @@
           playbooks/etcd-io-priority.yml once k3s is up.
       when:
         - _eio_scheduler | length > 0
-        - not _eio_k3s_cgroup.stat.exists
+        - not (_eio_k3s_cgroup.stat.exists | default(false))
 
     - name: Read back the live cgroup I/O weights
       ansible.builtin.command:
@@ -206,9 +206,13 @@
       changed_when: false
       check_mode: false
       failed_when: false
+      # `| default(false)`: Ansible templates `loop:` BEFORE evaluating `when:`, so on an NVMe
+      # node — where the stat task above is skipped and _eio_k3s_cgroup has no `.stat` at all —
+      # a bare `_eio_k3s_cgroup.stat.exists` raises even though this task will not run.
       loop: >-
         {{ ['system.slice']
-           + (['system.slice/k3s.service'] if _eio_k3s_cgroup.stat.exists else []) }}
+           + (['system.slice/k3s.service']
+              if (_eio_k3s_cgroup.stat.exists | default(false)) else []) }}
       when: _eio_scheduler | length > 0
 
     - name: VERIFY every live cgroup weight actually carries the boost