feat(domain,bff): current-registration lookup for self-service resume (refs #111)

Backend half of S-26. The domain gains IRegistrationStore.FindOpenByBsnAsync (the
citizen's non-terminal INGEDIEND/IN_BEHANDELING registration) + GET
/registrations/current?bsn=; the BFF adds owner-scoped GET /self-service/registrations
(bsn from the DigiD token) returning that registration or 204 when none. Regenerated
services/bff/openapi.json for the new endpoint (the api-client is generated from it).
Store + BFF endpoint unit tests; acceptance/BFF fakes updated for the new members.

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? CurrentQueriedBsn { get; private set; }
+
+    /// <summary>The current open registration the fake domain returns (null → the citizen has none in
+    /// flight, so the BFF replies 204). Tests set this to exercise resume.</summary>
+    public CurrentRegistration? Current { get; set; }
+
+    public Task<CurrentRegistration?> GetCurrentRegistrationAsync(string bsn, CancellationToken ct = default)
+    {
+        CurrentQueriedBsn = bsn;
+        return Task.FromResult(Current);
+    }
+
     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 Withdrawn { get; private set; }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

services/bff/Bff.Tests/SelfServiceEndpointTests.cs

@@ -1,6 +1,7 @@
 using System.Net;
 using System.Net.Http.Headers;
 using System.Net.Http.Json;
+using Bff.Api;
 
 namespace Bff.Tests;
 
@@ -168,5 +169,52 @@ public class SelfServiceEndpointTests
         Assert.Equal(HttpStatusCode.NotFound, response.StatusCode);
     }
 
+    private static HttpRequestMessage Current(string? bearer)
+    {
+        var request = new HttpRequestMessage(HttpMethod.Get, "/self-service/registrations");
+        if (bearer is not null)
+            request.Headers.Authorization = new AuthenticationHeaderValue("Bearer", bearer);
+        return request;
+    }
+
+    [Fact]
+    public async Task Rejects_the_current_registration_lookup_without_a_token()
+    {
+        using var factory = new BffFactory();
+
+        var response = await factory.CreateClient().SendAsync(Current(bearer: null));
+
+        Assert.Equal(HttpStatusCode.Unauthorized, response.StatusCode);
+    }
+
+    [Fact]
+    public async Task Returns_no_content_when_the_caller_has_no_open_registration()
+    {
+        using var factory = new BffFactory();
+        factory.Domain.Current = null;
+
+        var response = await factory.CreateClient().SendAsync(Current(TestTokens.Valid("123456782")));
+
+        Assert.Equal(HttpStatusCode.NoContent, response.StatusCode);
+        Assert.Equal("123456782", factory.Domain.CurrentQueriedBsn);
+    }
+
+    [Fact]
+    public async Task Returns_the_callers_current_registration_when_one_is_open()
+    {
+        using var factory = new BffFactory();
+        factory.Domain.Current = new CurrentRegistration("reg-77", "Ingediend");
+
+        var response = await factory.CreateClient().SendAsync(Current(TestTokens.Valid("123456782")));
+
+        Assert.Equal(HttpStatusCode.OK, response.StatusCode);
+        Assert.Equal("123456782", factory.Domain.CurrentQueriedBsn);
+        var body = await response.Content.ReadFromJsonAsync<CurrentRegistrationDto>();
+        Assert.Equal("reg-77", body!.RegistrationId);
+        Assert.Equal("Ingediend", body.Status);
+    }
+
     private sealed record SubmitAcceptedDto(string RegistrationId, string Status);
+
+    private sealed record CurrentRegistrationDto(string RegistrationId, string Status);
 }