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/Acceptance.csproj

@@ -11,6 +11,7 @@
     <PackageReference Include="coverlet.collector" Version="6.0.4" />
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.14.1" />
     <PackageReference Include="Reqnroll.xUnit" Version="3.3.4" />
+    <PackageReference Include="xunit" Version="2.9.3" />
     <PackageReference Include="xunit.runner.visualstudio" Version="3.1.4" />
   </ItemGroup>