CVE-2026-54526: Argo Workflows: ArtifactGC.PodSpecPatch bypasses Strict/Secure template reference allow-list (Incomplete fix for CVE-2026-31892)

The allow-list fix for CVE-2026-31892 (GHSA-3wf5-g532-rcrr), and its follow-up coverage of hostNetwork/securityContext/serviceAccountName in GHSA-3775-99mw-8rp4, is incomplete. workflow/util/merge.go ValidateUserOverrides / SanitizeUserWorkflowSpec walk only the top-level fields of WorkflowSpec via reflection. WorkflowSpec.ArtifactGC is allow-listed because admins want users to configure artifact garbage collection. The struct behind that field, WorkflowLevelArtifactGC, has a PodSpecPatch sub-field whose contents flow unmodified into util.ApplyPodSpecPatch on the artifact-GC pod - the same sink the original fix closed for WorkflowSpec.PodSpecPatch. A user submitting a Workflow under templateReferencing: Strict or Secure can therefore still inject an arbitrary strategic merge patch into the artifact-GC pod (hostPath volumes, privileged: true, arbitrary image and command, hostNetwork: true), defeating the stated purpose of Strict/Secure reference mode.