feat(zgw): OpenZaak Documenten (DRC) upload + zaak link (WP-51)

Extends the OpenZaak seam with IDocumentSource, sibling of IZaakSource
(WP-49/50): an upload always lands locally first (DocumentStore stays
the record of truth for preview/download/audit) and, when
Zgw:Enabled=true, is also registered as a DRC enkelvoudiginformatie-
object; once a zaak exists (IZaakSource.CreateZaak now also returns
its ZaakUrl), submit links each document to it via zaakinformatie-
object. FE upload/list DTOs are unchanged.

- ZgwOptions gains DrcBaseUrl + a category->informatieobjecttype URL
  map (the document analogue of ZaaktypeUrls).
- LocalDocumentSource is the same DocumentStore.Add/Link calls the
  endpoints used to make inline — zero behaviour change offline.
- OpenZaakDocumentSource POSTs the eio then the zaak link, persisting
  the DRC url (DocumentStore.SetDrcUrl) so linking doesn't re-upload.
- Factored the GET/POST-with-bearer-JWT plumbing shared with
  OpenZaakZaakSource into ZgwHttpClient; shared the stub handler
  between the two source test classes as ZgwStubHandler.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/tests/BigRegister.Tests/OpenZaakZaakSourceTests.cs

@@ -1,5 +1,3 @@
-using System.Net;
-using System.Text;
 using BigRegister.Api.Data;
 using BigRegister.Api.Zgw;
 
@@ -34,7 +32,7 @@ public class OpenZaakZaakSourceTests
   [Fact]
   public void Follows_pagination_caches_zaaktype_and_sends_bearer_token()
   {
-    var handler = new StubHandler(url => url switch
+    var handler = new ZgwStubHandler(url => url switch
     {
       _ when url == $"{ZrcBase}/zaken" => Page1,
       _ when url == $"{ZrcBase}/zaken?page=2" => Page2,
@@ -64,7 +62,7 @@ public class OpenZaakZaakSourceTests
   public void CreateZaak_posts_zaak_status_and_rol_and_maps_the_result_back()
   {
     const string zaaktypeUrl = $"{ZtBase}/zaaktypen/zt-registratie";
-    var handler = new StubHandler(url => url switch
+    var handler = new ZgwStubHandler(url => url switch
     {
       _ when url == $"{ZrcBase}/zaken" => $$"""
         { "url": "{{ZrcBase}}/zaken/uuid-new", "identificatie": "BIG-2026-000123",
@@ -103,27 +101,26 @@ public class OpenZaakZaakSourceTests
       Referentie = "BIG-2026-000123",
     };
 
-    var (referentie, status) = source.CreateZaak(aanvraag, new DateTimeOffset(2026, 7, 28, 12, 0, 0, TimeSpan.Zero));
+    var (referentie, status, zaakUrl) = source.CreateZaak(aanvraag, new DateTimeOffset(2026, 7, 28, 12, 0, 0, TimeSpan.Zero));
 
     Assert.Equal("BIG-2026-000123", referentie);
     Assert.Equal("InBehandeling", status.Tag);
     Assert.Equal("BIG-2026-000123", status.Referentie);
-
-    string BodyOf(string url) => handler.Bodies[handler.Requests.LastIndexOf(url)];
+    Assert.Equal($"{ZrcBase}/zaken/uuid-new", zaakUrl);
 
     // Zaak: mapped zaaktype + configured RSINs + the local reference as identificatie.
-    var zaakBody = BodyOf($"{ZrcBase}/zaken");
+    var zaakBody = handler.BodyOf($"{ZrcBase}/zaken");
     Assert.Contains(zaaktypeUrl, zaakBody);
     Assert.Contains("123443210", zaakBody);
     Assert.Contains("BIG-2026-000123", zaakBody);
 
     // Status: points at the created zaak's URL and the resolved statustype.
-    var statusBody = BodyOf($"{ZrcBase}/statussen");
+    var statusBody = handler.BodyOf($"{ZrcBase}/statussen");
     Assert.Contains($"{ZrcBase}/zaken/uuid-new", statusBody);
     Assert.Contains("statustypen/st-1", statusBody);
 
     // Rol: points at the created zaak, the resolved initiator roltype, and the BSN.
-    var rolBody = BodyOf($"{ZrcBase}/rollen");
+    var rolBody = handler.BodyOf($"{ZrcBase}/rollen");
     Assert.Contains($"{ZrcBase}/zaken/uuid-new", rolBody);
     Assert.Contains("roltypen/rt-initiator", rolBody);
     Assert.Contains("111222333", rolBody);
@@ -133,29 +130,10 @@ public class OpenZaakZaakSourceTests
   public void CreateZaak_throws_when_the_aanvraag_type_has_no_configured_zaaktype()
   {
     var options = new ZgwOptions { ZrcBaseUrl = ZrcBase, ZtcBaseUrl = ZtBase, ClientId = "c", Secret = "s" };
-    var handler = new StubHandler(url => throw new InvalidOperationException($"no HTTP call expected, got {url}"));
+    var handler = new ZgwStubHandler(url => throw new InvalidOperationException($"no HTTP call expected, got {url}"));
     var source = new OpenZaakZaakSource(new HttpClient(handler), new ZgwTokenProvider(options), options);
     var aanvraag = new Aanvraag { Id = "a1", Type = "unknown-type", Owner = "111222333", Referentie = "BIG-2026-000123" };
 
     Assert.Throws<InvalidOperationException>(() => source.CreateZaak(aanvraag, DateTimeOffset.UtcNow));
   }
-
-  private sealed class StubHandler(Func<string, string> respond) : HttpMessageHandler
-  {
-    public List<string> Requests { get; } = new();
-    public List<string?> AuthSchemes { get; } = new();
-    public List<string> Bodies { get; } = new();
-
-    protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
-    {
-      var url = request.RequestUri!.ToString();
-      Requests.Add(url);
-      AuthSchemes.Add(request.Headers.Authorization?.Scheme);
-      Bodies.Add(request.Content?.ReadAsStringAsync(cancellationToken).GetAwaiter().GetResult() ?? "");
-      return Task.FromResult(new HttpResponseMessage(HttpStatusCode.OK)
-      {
-        Content = new StringContent(respond(url), Encoding.UTF8, "application/json"),
-      });
-    }
-  }
 }