test(domain): a citizen can withdraw an open registration → INGETROKKEN (refs #12)

Withdraw is allowed from INGEDIEND or IN_BEHANDELING, needs no zaak, is idempotent, and is

rejected once the registration has been decided (INGESCHREVEN/AFGEWEZEN). The WithdrawRegistration

handler loads, withdraws, and persists; a repeated withdrawal is a no-op.

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

services/domain/Big.Tests/WithdrawRegistrationTests.cs

@@ -0,0 +1,60 @@
+using Big.Application;
+using Big.Domain;
+
+namespace Big.Tests;
+
+public class WithdrawRegistrationTests
+{
+    [Fact]
+    public async Task Withdrawing_marks_the_registration_ingetrokken_and_persists_it()
+    {
+        var store = new FakeRegistrationStore();
+        var registration = Registration.Submit("123456782");
+        store.Seed(registration);
+        var handler = new WithdrawRegistration(store);
+
+        await handler.HandleAsync(new WithdrawRegistrationCommand(registration.Id));
+
+        var saved = await store.GetAsync(registration.Id);
+        Assert.Equal(RegistrationStatus.Ingetrokken, saved!.Status);
+        Assert.Equal(1, store.SaveCount);
+    }
+
+    [Fact]
+    public async Task Rejects_a_null_command_without_touching_the_store()
+    {
+        var store = new FakeRegistrationStore();
+        var handler = new WithdrawRegistration(store);
+
+        await Assert.ThrowsAsync<ArgumentNullException>(() => handler.HandleAsync(null!));
+        Assert.Equal(0, store.SaveCount);
+    }
+
+    [Fact]
+    public async Task Withdrawing_an_unknown_registration_throws()
+    {
+        var store = new FakeRegistrationStore();
+        var handler = new WithdrawRegistration(store);
+
+        var ex = await Assert.ThrowsAsync<InvalidOperationException>(
+            () => handler.HandleAsync(new WithdrawRegistrationCommand(RegistrationId.New())));
+        Assert.Contains("No registration", ex.Message);
+        Assert.Equal(0, store.SaveCount);
+    }
+
+    [Fact]
+    public async Task Re_withdrawing_an_already_ingetrokken_registration_is_idempotent()
+    {
+        var store = new FakeRegistrationStore();
+        var registration = Registration.Submit("123456782");
+        store.Seed(registration);
+        var handler = new WithdrawRegistration(store);
+
+        await handler.HandleAsync(new WithdrawRegistrationCommand(registration.Id));
+        await handler.HandleAsync(new WithdrawRegistrationCommand(registration.Id));
+
+        // The second withdrawal is a no-op: the aggregate is not persisted again.
+        Assert.Equal(1, store.SaveCount);
+        Assert.Equal(RegistrationStatus.Ingetrokken, (await store.GetAsync(registration.Id))!.Status);
+    }
+}