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.Infrastructure/OpenZaakJobProcessor.cs

@@ -0,0 +1,26 @@
+using Big.Application;
+using Microsoft.Extensions.Logging;
+
+namespace Big.Infrastructure;
+
+/// <summary>
+/// One poll tick of the external-task worker (ADR-0009): acquire the parked <c>OpenZaakAanmaken</c>
+/// jobs, hand each to the <see cref="OpenZaakWorker"/> to open a zaak via the ACL, and complete the
+/// Flowable job with the resulting zaak URL. A job that fails is logged and left un-completed so
+/// Flowable redelivers it (§8.6). Split out from the hosted <see cref="OpenZaakJobPump"/> so the
+/// acquire→process→complete logic is unit-testable without a running host.
+/// </summary>
+public sealed class OpenZaakJobProcessor(
+    IExternalWorkerClient client,
+    OpenZaakWorker worker,
+    ILogger<OpenZaakJobProcessor> logger)
+{
+    /// <summary>Process up to <paramref name="maxJobs"/> jobs. Returns the number completed.</summary>
+    public async Task<int> PumpOnceAsync(int maxJobs, CancellationToken ct = default)
+    {
+        // STUB (red).
+        await Task.CompletedTask;
+        _ = logger;
+        return 0;
+    }
+}