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made a withdrawal advance the aggregate to INGETROKKEN; this sub-slice **cancels the running Flowable process** so the withdrawn case leaves the behandelaar's werkbak.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- **Werkbak**: also filters out registrations that are no longer open, so a withdrawn case never surfaces even in the brief window before cancellation lands.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- **verify (`run-domain-check.sh`)**: a second registration parks at `Beoordelen`, is withdrawn via the domain, and the check asserts its `Beoordelen` task disappears — so verify-stack validates the live Flowable message correlation.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/WithdrawRegistrationTests.cs

@@ -5,13 +5,20 @@ namespace Big.Tests;
 
 public class WithdrawRegistrationTests
 {
+    private static Registration Submitted(string processInstanceId = "proc-1")
+    {
+        var registration = Registration.Submit("123456782");
+        registration.RecordProcessStarted(processInstanceId);
+        return registration;
+    }
+
     [Fact]
     public async Task Withdrawing_marks_the_registration_ingetrokken_and_persists_it()
     {
         var store = new FakeRegistrationStore();
-        var registration = Registration.Submit("123456782");
+        var registration = Submitted();
         store.Seed(registration);
-        var handler = new WithdrawRegistration(store);
+        var handler = new WithdrawRegistration(store, new FakeWorkflowClient());
 
         await handler.HandleAsync(new WithdrawRegistrationCommand(registration.Id));
 
@@ -20,11 +27,43 @@ public class WithdrawRegistrationTests
         Assert.Equal(1, store.SaveCount);
     }
 
+    [Fact]
+    public async Task Withdrawing_cancels_the_running_process()
+    {
+        var store = new FakeRegistrationStore();
+        var registration = Submitted("proc-42");
+        store.Seed(registration);
+        var workflow = new FakeWorkflowClient();
+        var handler = new WithdrawRegistration(store, workflow);
+
+        await handler.HandleAsync(new WithdrawRegistrationCommand(registration.Id));
+
+        // The process the registration recorded at submit is cancelled (ADR-0014).
+        Assert.Equal("proc-42", workflow.WithdrawnProcessInstanceId);
+    }
+
+    [Fact]
+    public async Task Withdrawing_before_a_process_was_started_still_marks_ingetrokken()
+    {
+        // A registration with no recorded process (e.g. start never happened) can still be withdrawn;
+        // there is simply no process to cancel.
+        var store = new FakeRegistrationStore();
+        var registration = Registration.Submit("123456782"); // no RecordProcessStarted
+        store.Seed(registration);
+        var workflow = new FakeWorkflowClient();
+        var handler = new WithdrawRegistration(store, workflow);
+
+        await handler.HandleAsync(new WithdrawRegistrationCommand(registration.Id));
+
+        Assert.Equal(RegistrationStatus.Ingetrokken, (await store.GetAsync(registration.Id))!.Status);
+        Assert.Null(workflow.WithdrawnProcessInstanceId);
+    }
+
     [Fact]
     public async Task Rejects_a_null_command_without_touching_the_store()
     {
         var store = new FakeRegistrationStore();
-        var handler = new WithdrawRegistration(store);
+        var handler = new WithdrawRegistration(store, new FakeWorkflowClient());
 
         await Assert.ThrowsAsync<ArgumentNullException>(() => handler.HandleAsync(null!));
         Assert.Equal(0, store.SaveCount);
@@ -34,7 +73,7 @@ public class WithdrawRegistrationTests
     public async Task Withdrawing_an_unknown_registration_throws()
     {
         var store = new FakeRegistrationStore();
-        var handler = new WithdrawRegistration(store);
+        var handler = new WithdrawRegistration(store, new FakeWorkflowClient());
 
         var ex = await Assert.ThrowsAsync<InvalidOperationException>(
             () => handler.HandleAsync(new WithdrawRegistrationCommand(RegistrationId.New())));
@@ -46,9 +85,10 @@ public class WithdrawRegistrationTests
     public async Task Re_withdrawing_an_already_ingetrokken_registration_is_idempotent()
     {
         var store = new FakeRegistrationStore();
-        var registration = Registration.Submit("123456782");
+        var registration = Submitted();
         store.Seed(registration);
-        var handler = new WithdrawRegistration(store);
+        var workflow = new FakeWorkflowClient();
+        var handler = new WithdrawRegistration(store, workflow);
 
         await handler.HandleAsync(new WithdrawRegistrationCommand(registration.Id));
         await handler.HandleAsync(new WithdrawRegistrationCommand(registration.Id));