test(domain): approve use case sets the zaak status via the ACL, idempotently (refs #75)

Red: ApproveRegistration and IAclClient.ApproveZaakAsync don't exist yet.

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

services/domain/Big.Tests/Fakes.cs

@@ -52,10 +52,20 @@ internal sealed class FakeAclClient(Uri? zaakUrl = null) : IAclClient
     public string? OpenedForBsn { get; private set; }
     public int CallCount { get; private set; }
 
+    public Uri? ApprovedZaakUrl { get; private set; }
+    public int ApproveCallCount { get; private set; }
+
     public Task<Uri> OpenZaakAsync(string bsn, CancellationToken ct = default)
     {
         CallCount++;
         OpenedForBsn = bsn;
         return Task.FromResult(_zaakUrl);
     }
+
+    public Task ApproveZaakAsync(Uri zaakUrl, CancellationToken ct = default)
+    {
+        ApproveCallCount++;
+        ApprovedZaakUrl = zaakUrl;
+        return Task.CompletedTask;
+    }
 }