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.Api/DownstreamClients.cs

@@ -24,6 +24,9 @@ public interface IDomainClient
 
     /// <summary>The behandelaar's werkbak — registrations awaiting beoordeling.</summary>
     Task<IReadOnlyList<WerkbakItem>> GetWerkbakAsync(CancellationToken ct = default);
+
+    /// <summary>Apply a behandelaar's decision (<c>goedkeuren</c>/<c>afwijzen</c>) to a registration.</summary>
+    Task DecideAsync(string registrationId, string besluit, CancellationToken ct = default);
 }
 
 /// <summary>Port to the read projection.</summary>
@@ -47,6 +50,13 @@ public sealed class DomainClient(HttpClient http) : IDomainClient
     public async Task<IReadOnlyList<WerkbakItem>> GetWerkbakAsync(CancellationToken ct = default)
         => await http.GetFromJsonAsync<List<WerkbakItem>>("behandel/werkbak", ct) ?? [];
 
+    public async Task DecideAsync(string registrationId, string besluit, CancellationToken ct = default)
+    {
+        using var response = await http.PostAsJsonAsync(
+            $"registrations/{registrationId}/decide", new { besluit }, ct);
+        response.EnsureSuccessStatusCode();
+    }
+
     private sealed record DomainResponse(string RegistrationId, string Status, string? ZaakUrl);
 }