test(acl): OpenZaak default-fills ZGW fields and posts to OpenZaak (refs #5)

Add the ACL skeleton (Application/Infrastructure/Api per §9) and failing
xUnit tests for the single operation: AclService.OpenZaakAsync must
default-fill bronorganisatie, verantwoordelijkeOrganisatie, startdatum
(clock) and vertrouwelijkheidaanduiding and delegate to the gateway; the
OpenZaakGateway must POST the zaak to OpenZaak with a Bearer JWT and the
default-filled body. Implementations throw NotImplementedException — red.

Also add ADR-0003 (default-fill strategy).

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

services/acl/Acl.Tests/OpenZaakGatewayTests.cs

@@ -0,0 +1,51 @@
+using System.Net;
+using System.Net.Http.Json;
+using Acl.Application;
+using Acl.Infrastructure;
+
+namespace Acl.Tests;
+
+public class OpenZaakGatewayTests
+{
+    private sealed class StubHandler(Func<HttpRequestMessage, Task<HttpResponseMessage>> onSend)
+        : HttpMessageHandler
+    {
+        protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken ct)
+            => onSend(request);
+    }
+
+    [Fact]
+    public async Task Posts_zaak_to_openzaak_with_bearer_and_default_fields_and_returns_url()
+    {
+        HttpRequestMessage? seen = null;
+        string? body = null;
+        var handler = new StubHandler(async req =>
+        {
+            seen = req;
+            body = await req.Content!.ReadAsStringAsync();
+            return new HttpResponseMessage(HttpStatusCode.Created)
+            {
+                Content = JsonContent.Create(new { url = "http://openzaak/zaken/api/v1/zaken/xyz" }),
+            };
+        });
+        var gateway = new OpenZaakGateway(
+            new HttpClient(handler),
+            new OpenZaakOptions { BaseUrl = new("http://openzaak"), ClientId = "cid", Secret = "sec" });
+        var request = new ZaakRequest(
+            "517439943", "517439943", "openbaar",
+            new("http://openzaak/catalogi/api/v1/zaaktypen/big"), new DateOnly(2026, 6, 4));
+
+        var url = await gateway.OpenZaakAsync(request);
+
+        Assert.Equal("http://openzaak/zaken/api/v1/zaken/xyz", url.ToString());
+        Assert.Equal(HttpMethod.Post, seen!.Method);
+        Assert.Equal("http://openzaak/zaken/api/v1/zaken", seen.RequestUri!.ToString());
+        Assert.Equal("Bearer", seen.Headers.Authorization!.Scheme);
+        Assert.False(string.IsNullOrWhiteSpace(seen.Headers.Authorization.Parameter));
+        Assert.Contains("\"bronorganisatie\":\"517439943\"", body);
+        Assert.Contains("\"verantwoordelijkeOrganisatie\":\"517439943\"", body);
+        Assert.Contains("\"vertrouwelijkheidaanduiding\":\"openbaar\"", body);
+        Assert.Contains("\"startdatum\":\"2026-06-04\"", body);
+        Assert.Contains("\"zaaktype\":\"http://openzaak/catalogi/api/v1/zaaktypen/big\"", body);
+    }
+}