test(bff): self-service withdraw is owner-scoped and relays not-found (refs #12)

The withdraw handler returns an outcome and refuses a bsn that doesn't own the registration; the

BFF endpoint requires a digid token, forwards id+bsn, and relays the domain's 404.

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

services/bff/Bff.Tests/BffFactory.cs

@@ -82,6 +82,18 @@ internal sealed class FakeDomainClient : IDomainClient
         return Task.FromResult(Result);
     }
 
+    public (string RegistrationId, string Bsn)? Withdrawn { get; private set; }
+
+    /// <summary>Whether the fake domain reports the withdrawal as done (true → 204) or not-found/not-owned
+    /// (false → 404). Tests set this to exercise the relay.</summary>
+    public bool WithdrawSucceeds { get; set; } = true;
+
+    public Task<bool> WithdrawRegistrationAsync(string registrationId, string bsn, CancellationToken ct = default)
+    {
+        Withdrawn = (registrationId, bsn);
+        return Task.FromResult(WithdrawSucceeds);
+    }
+
     public (string RegistrationId, string Besluit)? Decided { get; private set; }
 
     public Task<IReadOnlyList<WerkbakItem>> GetWerkbakAsync(CancellationToken ct = default)