test(bff): /behandel/registrations/{id}/decide auth + forwarding + besluit validation (refs #13)

Red — the decide endpoint does not exist yet.

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

services/bff/Bff.Tests/BffFactory.cs

@@ -82,8 +82,16 @@ internal sealed class FakeDomainClient : IDomainClient
         return Task.FromResult(Result);
     }
 
+    public (string RegistrationId, string Besluit)? Decided { get; private set; }
+
     public Task<IReadOnlyList<WerkbakItem>> GetWerkbakAsync(CancellationToken ct = default)
         => Task.FromResult<IReadOnlyList<WerkbakItem>>(Werkbak);
+
+    public Task DecideAsync(string registrationId, string besluit, CancellationToken ct = default)
+    {
+        Decided = (registrationId, besluit);
+        return Task.CompletedTask;
+    }
 }
 
 /// <summary>Serves a configurable set of projection rows.</summary>