test(acl): BDD acceptance scenario for opening a zaak (closes #5) (#49)

tests/acceptance/Support/FixedClock.cs

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+using Acl.Application;
+
+namespace Acceptance.Support;
+
+/// <summary>A clock pinned to a known date so <c>startdatum</c> is deterministic
+/// in scenarios (ADR-0003).</summary>
+public sealed class FixedClock(DateOnly today) : IClock
+{
+    public DateOnly Today { get; } = today;
+}

tests/acceptance/Support/InMemoryZaakGateway.cs

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+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);
+    }
+}