test(bff): acceptance scenario for BFF access (valid/invalid tokens) (refs #8)

Use-case-level BDD (Reqnroll) driving the real BFF over HTTP with fake downstreams
and locally-minted tokens: a valid DigiD token is accepted and the bsn forwarded
to the domain; a tokenless submit is 401; the openbaar register is anonymous and
never exposes the bsn (ADR-0010). Real Keycloak validation is the verify-bff check.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/acceptance/Acceptance.csproj

@@ -9,6 +9,8 @@
 
   <ItemGroup>
     <PackageReference Include="coverlet.collector" Version="6.0.4" />
+    <!-- The BFF acceptance scenario drives the real HTTP endpoints in-process (ADR-0010). -->
+    <PackageReference Include="Microsoft.AspNetCore.Mvc.Testing" Version="10.0.8" />
     <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" />
@@ -20,6 +22,7 @@
     <ProjectReference Include="..\..\services\acl\Acl.Infrastructure\Acl.Infrastructure.csproj" />
     <ProjectReference Include="..\..\services\event-subscriber\EventSubscriber.Application\EventSubscriber.Application.csproj" />
     <ProjectReference Include="..\..\services\domain\Big.Application\Big.Application.csproj" />
+    <ProjectReference Include="..\..\services\bff\Bff.Api\Bff.Api.csproj" />
   </ItemGroup>
 
 </Project>