fix: address PR #69 review feedback

- VmService.ExecuteGuestCommand: guard against null elements in -Args.
  The old JSON-serialization tolerated nulls (as "null"); the repeated-key
  path would NRE in EncodeFormValue. Throw a clear ArgumentException instead.
- findings.json: refresh the stale counters block (untouched since F085) to
  the actual ledger state — next_id 92, resolved 83 — and bump last_updated
  to 2026-05-22. last_scan_date stays 2026-03-26 (F086–F091 came from issue
  triage, not a formal review scan).
- VmServiceTests: add empty-array (single command entry) and null-element
  (throws) cases.

Note: F091 is the correct next ID — F086–F090 already exist from prior
merged PRs (#60/#61/#66/#67); only the counters were lagging.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/review/findings.json

@@ -1,15 +1,15 @@
 {
   "_schema_version": "1.0",
   "_description": "PSProxmoxVE stable findings ledger. IDs are permanent (F001, F002...). Resolved findings are never deleted — they are marked resolved with evidence. If a finding reappears, it is marked regressed and retains its original ID.",
-  "last_updated": "2026-03-26",
+  "last_updated": "2026-05-22",
   "last_scan_date": "2026-03-26",
   "counters": {
-    "next_id": 86,
+    "next_id": 92,
     "total_open": 7,
-    "total_resolved": 77,
+    "total_resolved": 83,
     "total_regressed": 0,
     "open": 7,
-    "resolved": 77,
+    "resolved": 83,
     "regressed": 0,
     "wont_fix": 1
   },