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/FlowableWorkflowClientTests.cs

@@ -158,14 +158,13 @@ public class FlowableWorkflowClientTests
         var capture = new RequestCapture();
         var client = Client(capture.Responds(HttpStatusCode.OK,
             $$"""
-            {"data":[{"id":"task-1","executionId":"exec-1","variables":[{"name":"registrationId","type":"string","value":"{{rid}}"}]}],"total":1}
+            {"data":[{"id":"task-1","variables":[{"name":"registrationId","type":"string","value":"{{rid}}"}]}],"total":1}
             """));
 
         var tasks = await client.GetOpenBeoordelingenAsync();
 
         var task = Assert.Single(tasks);
         Assert.Equal("task-1", task.TaskId);
-        Assert.Equal("exec-1", task.ExecutionId);
         Assert.Equal(rid, task.RegistrationId);
         Assert.Equal(HttpMethod.Post, capture.Seen!.Method);
         Assert.Equal("http://flowable/flowable-rest/service/query/tasks",
@@ -244,29 +243,58 @@ public class FlowableWorkflowClientTests
     }
 
     [Fact]
-    public async Task Withdraw_beoordeling_delivers_the_message_to_the_task_execution()
+    public async Task Withdraw_process_correlates_the_message_to_the_subscribed_execution()
     {
-        var capture = new RequestCapture();
-        var client = Client(capture.Responds(HttpStatusCode.OK));
+        HttpRequestMessage? getReq = null;
+        HttpRequestMessage? putReq = null;
+        string? putBody = null;
+        var client = Client(new StubHandler(async req =>
+        {
+            if (req.Method == HttpMethod.Get)
+            {
+                getReq = req;
+                return new HttpResponseMessage(HttpStatusCode.OK)
+                {
+                    Content = new StringContent("""{"data":[{"id":"exec-9"}]}""", Encoding.UTF8, "application/json"),
+                };
+            }
+            putReq = req;
+            putBody = await req.Content!.ReadAsStringAsync();
+            return new HttpResponseMessage(HttpStatusCode.OK);
+        }));
 
-        await client.WithdrawBeoordelingAsync("exec-1");
+        await client.WithdrawProcessAsync("pi-1");
 
-        // A PUT messageEventReceived on the execution trips the Beoordelen boundary event (ADR-0014).
-        Assert.Equal(HttpMethod.Put, capture.Seen!.Method);
-        Assert.Equal("http://flowable/flowable-rest/service/runtime/executions/exec-1",
-            capture.Seen.RequestUri!.ToString());
-        Assert.Equal("rest-admin:test", DecodeBasic(capture.Seen));
-        Assert.Contains("\"action\":\"messageEventReceived\"", capture.Body);
-        Assert.Contains("\"messageName\":\"RegistratieIngetrokken\"", capture.Body);
+        // 1. Find the execution subscribed to the withdrawal message for this instance.
+        Assert.Equal(HttpMethod.Get, getReq!.Method);
+        Assert.Contains("service/runtime/executions", getReq.RequestUri!.ToString());
+        Assert.Contains("messageEventSubscriptionName=RegistratieIngetrokken", getReq.RequestUri!.Query);
+        Assert.Contains("processInstanceId=pi-1", getReq.RequestUri!.Query);
+        // 2. Deliver messageEventReceived to that execution (PUT), tripping the interrupting event.
+        Assert.Equal(HttpMethod.Put, putReq!.Method);
+        Assert.Equal("http://flowable/flowable-rest/service/runtime/executions/exec-9",
+            putReq.RequestUri!.ToString());
+        Assert.Contains("\"action\":\"messageEventReceived\"", putBody);
+        Assert.Contains("\"messageName\":\"RegistratieIngetrokken\"", putBody);
     }
 
     [Fact]
-    public async Task Withdraw_beoordeling_throws_when_flowable_rejects_the_request()
+    public async Task Withdraw_process_is_a_no_op_when_no_execution_is_subscribed()
     {
-        var capture = new RequestCapture();
-        var client = Client(capture.Responds(HttpStatusCode.InternalServerError));
+        var methods = new List<HttpMethod>();
+        var client = Client(new StubHandler(req =>
+        {
+            methods.Add(req.Method);
+            return Task.FromResult(new HttpResponseMessage(HttpStatusCode.OK)
+            {
+                Content = new StringContent("""{"data":[]}""", Encoding.UTF8, "application/json"),
+            });
+        }));
 
-        await Assert.ThrowsAsync<HttpRequestException>(() => client.WithdrawBeoordelingAsync("exec-1"));
+        await client.WithdrawProcessAsync("pi-1");
+
+        // No subscribed execution → no message delivered (best-effort), no throw.
+        Assert.DoesNotContain(HttpMethod.Put, methods);
     }
 
     [Fact]