feat(domain): withdrawal cancels the registratie process (S-11b, refs #12) (#89)

## What & why

Second sub-slice of **S-11 · Withdrawal (Flow 3)** (#12). S-11a (#88) made a withdrawal advance the aggregate to INGETROKKEN; this sub-slice **cancels the running Flowable process** so the withdrawn case leaves the behandelaar's werkbak.

- **BPMN** (`registratie.bpmn`): an interrupting message boundary event (`RegistratieIngetrokken`) on the `Beoordelen` task, routing to a dedicated "Registratie ingetrokken" end event.
- **Workflow Client**: `WithdrawBeoordelingAsync(executionId)` delivers `messageEventReceived` to the task's execution (PUT); `BeoordelingTask` now carries its `executionId`.
- **`WithdrawRegistration` handler**: after the domain transition, finds the open `Beoordelen` task for the registration and delivers the withdrawal message — best-effort, mirroring how the beoordeling completes its task.
- **Werkbak**: also filters out registrations that are no longer open, so a withdrawn case never surfaces even in the brief window before cancellation lands.
- **ADR-0014** records the decision (message event in BPMN vs. deleting the instance from code).
- **verify (`run-domain-check.sh`)**: a second registration parks at `Beoordelen`, is withdrawn via the domain, and the check asserts its `Beoordelen` task disappears — so verify-stack validates the live Flowable message correlation.

Refs #12 (S-11c — the BFF + self-service "trek aanvraag in" button + e2e — closes it).

## Definition of Done

- [x] Linked Gitea issue (#12).
- [x] Failing tests committed before the implementation (red → green per commit).
- [x] Implementation makes the tests pass.
- [x] Conventional Commits referencing the issue (`refs #12`).
- [ ] CI green — all Gitea Actions jobs.
- [x] `docker compose up` unaffected (BPMN redeploys on a fresh CI DB via flowable-init).
- [x] ADR added (ADR-0014).
- [x] Docs — the user-visible demo note lands with S-11c.

## Notes for reviewers

- Verified locally: `Big.Tests` 94/94 pass; `Big.Api` builds; `registratie.bpmn` is well-formed.
- The Flowable message-correlation REST shape is validated **live** by verify-stack (the Workflow Client unit tests stub the exchange and assert only the request shape, per ADR-0009) — the new `run-domain-check.sh` withdrawal step is that live check.
- Known gap (ADR-0014): a withdrawal that races ahead of the process reaching `Beoordelen` finds no task to cancel; the aggregate is still INGETROKKEN and the werkbak filter hides it, but that instance parks unattended. A process-level event subprocess would close the gap — deferred.

Reviewed-on: #89

services/domain/Big.Tests/FlowableWorkflowClientTests.cs

@@ -242,6 +242,61 @@ public class FlowableWorkflowClientTests
         Assert.Contains($"\"value\":\"{expected}\"", capture.Body);
     }
 
+    [Fact]
+    public async Task Withdraw_process_correlates_the_message_to_the_subscribed_execution()
+    {
+        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.WithdrawProcessAsync("pi-1");
+
+        // 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_process_is_a_no_op_when_no_execution_is_subscribed()
+    {
+        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 client.WithdrawProcessAsync("pi-1");
+
+        // No subscribed execution → no message delivered (best-effort), no throw.
+        Assert.DoesNotContain(HttpMethod.Put, methods);
+    }
+
     [Fact]
     public async Task Complete_beoordeling_throws_when_flowable_rejects_the_request()
     {