test(domain): Workflow Client, ACL client, store and job processor (refs #6)

Failing infrastructure unit tests (stub HttpMessageHandler, fakes):
- FlowableWorkflowClient starts a process with the registrationId variable and
  returns the instance id; acquires OpenZaakAanmaken jobs (topic/workerId/lock)
  and parses their registrationId; completes a job with the zaakUrl variable —
  request URIs match flowable-rest's service/ and external-job-api/ paths.
- AclHttpClient POSTs the bsn to the ACL and returns the zaak URL.
- InMemoryRegistrationStore saves/reads/upserts by id.
- OpenZaakJobProcessor acquires, opens a zaak, completes the job; leaves a failing
  job uncompleted for redelivery; polls harmlessly when idle.

Adapters are stubs so the tests compile and fail on their assertions; the green
commit implements them against the REST contract verified on a live Flowable.

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

services/domain/Big.Tests/HttpStubs.cs

@@ -0,0 +1,30 @@
+using System.Net;
+
+namespace Big.Tests;
+
+/// <summary>A test double for <see cref="HttpMessageHandler"/> that records the last request and
+/// returns a scripted response — the same approach the ACL gateway tests use.</summary>
+internal sealed class StubHandler(Func<HttpRequestMessage, Task<HttpResponseMessage>> onSend) : HttpMessageHandler
+{
+    protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken ct)
+        => onSend(request);
+}
+
+/// <summary>Captures the request a client sent, including the (buffered) body.</summary>
+internal sealed class RequestCapture
+{
+    public HttpRequestMessage? Seen { get; private set; }
+    public string? Body { get; private set; }
+
+    /// <summary>A handler that records the request, then replies with <paramref name="status"/> and
+    /// <paramref name="json"/>.</summary>
+    public StubHandler Responds(HttpStatusCode status, string? json = null) => new(async req =>
+    {
+        Seen = req;
+        Body = req.Content is null ? null : await req.Content.ReadAsStringAsync();
+        var response = new HttpResponseMessage(status);
+        if (json is not null)
+            response.Content = new StringContent(json, System.Text.Encoding.UTF8, "application/json");
+        return response;
+    });
+}