fix(test): implement ListZaaktypenAsync in the acceptance InMemoryZaakGateway (refs #130)

tests/acceptance/Support/InMemoryZaakGateway.cs

@@ -48,4 +48,8 @@ public sealed class InMemoryZaakGateway : IZaakGateway
 
     public Task<Uri> ResolveInformatieobjecttypeUrlAsync(string omschrijving, CancellationToken ct = default)
         => Task.FromResult(ResolvedInformatieobjecttypeUrl);
+
+    public Task<IReadOnlyList<ZaaktypeSummary>> ListZaaktypenAsync(CancellationToken ct = default)
+        => Task.FromResult<IReadOnlyList<ZaaktypeSummary>>(
+            [new ZaaktypeSummary("BIG-REGISTRATIE", "BIG-registratie", ResolvedZaaktypeUrl)]);
 }