fix(acceptance): implement WithdrawBeoordelingAsync on the in-memory user-task client (refs #12)

The S-11b port change (WithdrawBeoordelingAsync + BeoordelingTask.ExecutionId) broke the solution

build: the acceptance in-memory IUserTaskClient still used the old signature. Implement the new

member and carry an executionId.

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

tests/acceptance/Support/InMemoryDomainPorts.cs

@@ -50,8 +50,10 @@ public sealed class InMemoryUserTaskClient : IUserTaskClient
     private readonly List<BeoordelingTask> _open = [];
 
     public (string TaskId, BeoordelingsBesluit Besluit)? Completed { get; private set; }
+    public string? WithdrawnExecutionId { get; private set; }
 
-    public void Open(RegistrationId registrationId) => _open.Add(new BeoordelingTask($"task-{registrationId}", registrationId));
+    public void Open(RegistrationId registrationId) =>
+        _open.Add(new BeoordelingTask($"task-{registrationId}", $"exec-{registrationId}", registrationId));
 
     public Task<IReadOnlyList<BeoordelingTask>> GetOpenBeoordelingenAsync(CancellationToken ct = default)
         => Task.FromResult<IReadOnlyList<BeoordelingTask>>(_open);
@@ -64,6 +66,13 @@ public sealed class InMemoryUserTaskClient : IUserTaskClient
         _open.RemoveAll(t => t.TaskId == taskId);
         return Task.CompletedTask;
     }
+
+    public Task WithdrawBeoordelingAsync(string executionId, CancellationToken ct = default)
+    {
+        WithdrawnExecutionId = executionId;
+        _open.RemoveAll(t => t.ExecutionId == executionId);
+        return Task.CompletedTask;
+    }
 }
 
 /// <summary>An in-memory registration store for the domain acceptance scenario.</summary>