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>

src/PSProxmoxVE.Core/Services/VmService.cs

@@ -610,7 +610,11 @@ namespace PSProxmoxVE.Core.Services
                 if (args != null)
                 {
                     foreach (var arg in args)
+                    {
+                        if (arg == null)
+                            throw new ArgumentException("Args elements must not be null.", nameof(args));
                         data.Add(new KeyValuePair<string, string>("command", arg));
+                    }
                 }
 
                 var response = client.PostAsync($"nodes/{Uri.EscapeDataString(node)}/qemu/{vmid}/agent/exec", data)