feat(acl): wire BDD step bindings to AclService (refs #5)

Implement the step definitions for EenZaakOpenen.feature against the
existing AclService, with an in-memory gateway standing in for the OpenZaak
Zaken API. The scenario now passes: a domain registration is default-filled
into a ZaakRequest (ADR-0003) and the created zaak URL is returned.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/acceptance/Support/InMemoryZaakGateway.cs

@@ -0,0 +1,20 @@
+using Acl.Application;
+
+namespace Acceptance.Support;
+
+/// <summary>An in-memory stand-in for the OpenZaak Zaken API. It captures the
+/// fully default-filled <see cref="ZaakRequest"/> the ACL builds and returns a
+/// fixed zaak URL, so the acceptance scenario can verify the use case without a
+/// running OpenZaak (real-OpenZaak verification is a separate slice).</summary>
+public sealed class InMemoryZaakGateway : IZaakGateway
+{
+    public static readonly Uri CreatedZaakUrl = new("http://openzaak/zaken/api/v1/zaken/created-123");
+
+    public ZaakRequest? Captured { get; private set; }
+
+    public Task<Uri> OpenZaakAsync(ZaakRequest request, CancellationToken ct = default)
+    {
+        Captured = request;
+        return Task.FromResult(CreatedZaakUrl);
+    }
+}