fix(domain): correlate withdrawal by process instance to the subscribed execution (refs #12)

verify-stack surfaced a Flowable 500: delivering messageEventReceived to the Beoordelen task's

execution is wrong — a message boundary event's subscription lives on its own execution. Correlate

instead by the registration's process instance id (recorded at submit): query the execution

subscribed to RegistratieIngetrokken and deliver the message there. Withdrawal moves from

IUserTaskClient to IWorkflowClient.WithdrawProcessAsync; the handler no longer needs a task lookup.

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

tests/acceptance/Support/InMemoryDomainPorts.cs

@@ -11,12 +11,19 @@ public sealed class InMemoryWorkflowClient : IWorkflowClient
     public const string StartedProcessInstanceId = "proc-acc-1";
 
     public RegistrationId? StartedFor { get; private set; }
+    public string? WithdrawnProcessInstanceId { get; private set; }
 
     public Task<string> StartRegistrationProcessAsync(RegistrationId registrationId, CancellationToken ct = default)
     {
         StartedFor = registrationId;
         return Task.FromResult(StartedProcessInstanceId);
     }
+
+    public Task WithdrawProcessAsync(string processInstanceId, CancellationToken ct = default)
+    {
+        WithdrawnProcessInstanceId = processInstanceId;
+        return Task.CompletedTask;
+    }
 }
 
 /// <summary>An in-memory ACL stand-in: records the bsn it opened a zaak for and returns a fixed URL,
@@ -50,10 +57,8 @@ 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}", $"exec-{registrationId}", registrationId));
+    public void Open(RegistrationId registrationId) => _open.Add(new BeoordelingTask($"task-{registrationId}", registrationId));
 
     public Task<IReadOnlyList<BeoordelingTask>> GetOpenBeoordelingenAsync(CancellationToken ct = default)
         => Task.FromResult<IReadOnlyList<BeoordelingTask>>(_open);
@@ -66,13 +71,6 @@ 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>