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>

services/domain/Big.Tests/WerkbakTests.cs

@@ -18,7 +18,7 @@ public class WerkbakTests
         registration.AttachZaak(FakeAclClient.DefaultZaakUrl);
         registration.TakeIntoBehandeling();
         store.Seed(registration);
-        var tasks = new FakeUserTaskClient([new BeoordelingTask("task-1", "exec-1", registration.Id)]);
+        var tasks = new FakeUserTaskClient([new BeoordelingTask("task-1", registration.Id)]);
         var werkbak = new Werkbak(tasks, store);
 
         var items = await werkbak.GetAsync();
@@ -41,7 +41,7 @@ public class WerkbakTests
     public async Task Skips_a_task_whose_registration_is_unknown()
     {
         // Defensive: the werkbak never invents an item for a task the domain has no aggregate for.
-        var tasks = new FakeUserTaskClient([new BeoordelingTask("task-1", "exec-1", RegistrationId.New())]);
+        var tasks = new FakeUserTaskClient([new BeoordelingTask("task-1", RegistrationId.New())]);
         var werkbak = new Werkbak(tasks, new FakeRegistrationStore());
 
         Assert.Empty(await werkbak.GetAsync());
@@ -56,7 +56,7 @@ public class WerkbakTests
         var registration = Registration.Submit("123456782");
         registration.Withdraw();
         store.Seed(registration);
-        var tasks = new FakeUserTaskClient([new BeoordelingTask("task-1", "exec-1", registration.Id)]);
+        var tasks = new FakeUserTaskClient([new BeoordelingTask("task-1", registration.Id)]);
         var werkbak = new Werkbak(tasks, store);
 
         Assert.Empty(await werkbak.GetAsync());