test(domain): SubmitRegistration + OpenZaakWorker use cases (refs #6)

Failing application-layer tests over fake ports (IWorkflowClient, IAclClient,
IRegistrationStore):
- Submit persists an INGEDIEND registration and starts the registratie process,
  recording the instance id — and persists *before* starting, so the worker can
  correlate the OpenZaakAanmaken job back to its aggregate (ADR-0009).
- The worker opens a zaak via the ACL and attaches it; an unknown registration
  throws (job left for redelivery); a redelivered job is idempotent and opens no
  second zaak (§8.6).

Handlers are stubs (no persistence / no ACL call) so the tests compile and fail
on their assertions; the green commit implements them.

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

services/domain/Big.Tests/SubmitRegistrationTests.cs

@@ -0,0 +1,52 @@
+using Big.Application;
+using Big.Domain;
+
+namespace Big.Tests;
+
+public class SubmitRegistrationTests
+{
+    [Fact]
+    public async Task Submitting_persists_an_ingediend_registration_and_starts_the_process()
+    {
+        var store = new InMemoryRegistrationStore();
+        var workflow = new FakeWorkflowClient(processInstanceId: "proc-42");
+        var handler = new SubmitRegistration(store, workflow);
+
+        var id = await handler.HandleAsync(new SubmitRegistrationCommand("123456782"));
+
+        var saved = await store.GetAsync(id);
+        Assert.NotNull(saved);
+        Assert.Equal("123456782", saved.Bsn);
+        Assert.Equal(RegistrationStatus.Ingediend, saved.Status);
+        Assert.Equal("proc-42", saved.ProcessInstanceId);
+        Assert.Equal(id, workflow.StartedFor);
+        Assert.Null(saved.ZaakUrl);
+    }
+
+    [Fact]
+    public async Task Submitting_persists_the_registration_before_starting_the_process()
+    {
+        // The worker correlates the OpenZaakAanmaken job back to the aggregate by id, so the
+        // aggregate must already be persisted when the process starts (ADR-0009).
+        var store = new InMemoryRegistrationStore();
+        Registration? visibleAtStart = null;
+        var workflow = new FakeWorkflowClient(onStart: id => visibleAtStart = store.GetAsync(id).Result);
+        var handler = new SubmitRegistration(store, workflow);
+
+        await handler.HandleAsync(new SubmitRegistrationCommand("123456782"));
+
+        Assert.NotNull(visibleAtStart);
+    }
+
+    [Fact]
+    public async Task Submitting_without_a_bsn_is_rejected_and_starts_no_process()
+    {
+        var store = new InMemoryRegistrationStore();
+        var workflow = new FakeWorkflowClient();
+        var handler = new SubmitRegistration(store, workflow);
+
+        await Assert.ThrowsAnyAsync<ArgumentException>(
+            () => handler.HandleAsync(new SubmitRegistrationCommand("")));
+        Assert.Null(workflow.StartedFor);
+    }
+}