test(domain): expiry worker cancels the zaak via the ACL on timeout (refs #106)

S-10c: expiring the aggregate to VERLOPEN is not enough — the ZGW zaak must
also be cancelled through the ACL (§8.1). Adds IAclClient.CancelZaakAsync and
its client/fakes, and asserts the worker cancels a still-open registration's
zaak but leaves an already-resolved one untouched.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

services/domain/Big.Tests/Fakes.cs

@@ -119,4 +119,14 @@ internal sealed class FakeAclClient(Uri? zaakUrl = null) : IAclClient
         StoredDiploma = (zaakUrl, content, fileName, contentType);
         return Task.FromResult(DefaultDocumentUrl);
     }
+
+    public Uri? CancelledZaakUrl { get; private set; }
+    public int CancelCallCount { get; private set; }
+
+    public Task CancelZaakAsync(Uri zaakUrl, CancellationToken ct = default)
+    {
+        CancelCallCount++;
+        CancelledZaakUrl = zaakUrl;
+        return Task.CompletedTask;
+    }
 }