test(domain): acceptance scenario for submitting a registration (refs #6)

Use-case-level BDD (Reqnroll) for S-05: a zorgprofessional submits a registration;
the Domain Service starts the registratie process and the OpenZaakAanmaken external
task opens a zaak via the ACL, recorded on the aggregate (ADR-0009). Driven against
in-memory Workflow Client and ACL stand-ins; real Flowable+ACL+OpenZaak delivery is
the live-stack verify-domain check.

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

tests/acceptance/Steps/RegistratieIndienenSteps.cs

@@ -0,0 +1,54 @@
+using Acceptance.Support;
+using Big.Application;
+using Big.Domain;
+using Reqnroll;
+using Xunit;
+
+namespace Acceptance.Steps;
+
+/// <summary>Bindings for <c>RegistratieIndienen.feature</c> (S-05). Drives the SubmitRegistration
+/// and OpenZaakWorker use cases against in-memory ports; one instance per scenario.</summary>
+[Binding]
+public sealed class RegistratieIndienenSteps
+{
+    private readonly InMemoryWorkflowClient _workflow = new();
+    private readonly InMemoryAclClient _acl = new();
+    private readonly InMemoryRegistrationStore _store = new();
+    private string _bsn = "";
+    private RegistrationId _id;
+
+    [Given("a zorgprofessional submits a registration for BSN \"(.*)\"")]
+    public void GivenAZorgprofessionalSubmitsARegistration(string bsn) => _bsn = bsn;
+
+    [When("the domain service handles the submission")]
+    public async Task WhenTheDomainServiceHandlesTheSubmission()
+        => _id = await new SubmitRegistration(_store, _workflow).HandleAsync(new SubmitRegistrationCommand(_bsn));
+
+    [Then("a registratie process is started for the registration")]
+    public void ThenARegistratieProcessIsStarted()
+        => Assert.Equal(_id, _workflow.StartedFor);
+
+    [Then("the registration has status \"(.*)\"")]
+    public async Task ThenTheRegistrationHasStatus(string expected)
+    {
+        var registration = await _store.GetAsync(_id);
+        Assert.NotNull(registration);
+        Assert.Equal(expected, registration.Status.ToString().ToUpperInvariant());
+        Assert.Equal(InMemoryWorkflowClient.StartedProcessInstanceId, registration.ProcessInstanceId);
+    }
+
+    [When("the OpenZaakAanmaken external task is handled for the registration")]
+    public async Task WhenTheExternalTaskIsHandled()
+        => await new OpenZaakWorker(_store, _acl).HandleAsync(new OpenZaakJob("job-acc-1", _id));
+
+    [Then("a zaak is opened via the ACL for BSN \"(.*)\"")]
+    public void ThenAZaakIsOpenedViaTheAcl(string bsn)
+        => Assert.Equal(bsn, _acl.OpenedForBsn);
+
+    [Then("the zaak is recorded on the registration")]
+    public async Task ThenTheZaakIsRecordedOnTheRegistration()
+    {
+        var registration = await _store.GetAsync(_id);
+        Assert.Equal(InMemoryAclClient.OpenedZaakUrl, registration!.ZaakUrl);
+    }
+}