test(domain): withdrawal cancels the workflow via the Beoordelen task's execution (refs #12)

The WithdrawRegistration handler delivers the withdrawal message to the open Beoordelen task's

execution; the Workflow Client PUTs messageEventReceived (RegistratieIngetrokken). BeoordelingTask

now carries its executionId.

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

services/domain/Big.Tests/Fakes.cs

@@ -47,6 +47,7 @@ internal sealed class FakeUserTaskClient(IReadOnlyList<BeoordelingTask> open) :
 {
     public (string TaskId, string Behandelaar)? Claimed { get; private set; }
     public (string TaskId, BeoordelingsBesluit Besluit)? Completed { get; private set; }
+    public string? WithdrawnExecutionId { get; private set; }
 
     public Task<IReadOnlyList<BeoordelingTask>> GetOpenBeoordelingenAsync(CancellationToken ct = default)
         => Task.FromResult(open);
@@ -62,6 +63,12 @@ internal sealed class FakeUserTaskClient(IReadOnlyList<BeoordelingTask> open) :
         Completed = (taskId, besluit);
         return Task.CompletedTask;
     }
+
+    public Task WithdrawBeoordelingAsync(string executionId, CancellationToken ct = default)
+    {
+        WithdrawnExecutionId = executionId;
+        return Task.CompletedTask;
+    }
 }
 
 /// <summary>A fake ACL client that records the bsn it was asked to open a zaak for and returns a