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/OpenZaakJobProcessorTests.cs

@@ -0,0 +1,77 @@
+using Big.Application;
+using Big.Domain;
+using Big.Infrastructure;
+using Microsoft.Extensions.Logging.Abstractions;
+
+namespace Big.Tests;
+
+public class OpenZaakJobProcessorTests
+{
+    /// <summary>A fake external-worker client: scripts the jobs to acquire and records completions.</summary>
+    private sealed class FakeExternalWorkerClient(params OpenZaakJob[] jobs) : IExternalWorkerClient
+    {
+        public int AcquireCount { get; private set; }
+        public List<(string JobId, Uri ZaakUrl)> Completed { get; } = [];
+
+        public Task<IReadOnlyList<OpenZaakJob>> AcquireOpenZaakJobsAsync(int maxJobs, CancellationToken ct = default)
+        {
+            AcquireCount++;
+            return Task.FromResult<IReadOnlyList<OpenZaakJob>>(jobs.Take(maxJobs).ToList());
+        }
+
+        public Task CompleteOpenZaakJobAsync(string jobId, Uri zaakUrl, CancellationToken ct = default)
+        {
+            Completed.Add((jobId, zaakUrl));
+            return Task.CompletedTask;
+        }
+    }
+
+    private static OpenZaakJobProcessor Processor(IExternalWorkerClient client, IRegistrationStore store, FakeAclClient acl)
+        => new(client, new OpenZaakWorker(store, acl), NullLogger<OpenZaakJobProcessor>.Instance);
+
+    [Fact]
+    public async Task Acquires_a_job_opens_the_zaak_and_completes_it()
+    {
+        var registration = Registration.Submit("123456782");
+        var store = new FakeRegistrationStore();
+        store.Seed(registration);
+        var client = new FakeExternalWorkerClient(new OpenZaakJob("job-1", registration.Id));
+        var acl = new FakeAclClient();
+
+        var acquired = await Processor(client, store, acl).PumpOnceAsync(5);
+
+        Assert.Equal(1, acquired);
+        var completed = Assert.Single(client.Completed);
+        Assert.Equal("job-1", completed.JobId);
+        Assert.Equal(FakeAclClient.DefaultZaakUrl, completed.ZaakUrl);
+        Assert.Equal(FakeAclClient.DefaultZaakUrl, (await store.GetAsync(registration.Id))!.ZaakUrl);
+    }
+
+    [Fact]
+    public async Task A_failing_job_is_left_uncompleted_for_flowable_to_redeliver()
+    {
+        var store = new FakeRegistrationStore();
+        var acl = new FakeAclClient();
+        // The job correlates to a registration that is not in the store, so the worker throws.
+        var client = new FakeExternalWorkerClient(new OpenZaakJob("job-1", RegistrationId.New()));
+
+        var acquired = await Processor(client, store, acl).PumpOnceAsync(5);
+
+        Assert.Equal(1, acquired);
+        Assert.Empty(client.Completed);
+    }
+
+    [Fact]
+    public async Task Does_nothing_but_poll_when_there_are_no_jobs()
+    {
+        var store = new FakeRegistrationStore();
+        var acl = new FakeAclClient();
+        var client = new FakeExternalWorkerClient();
+
+        var acquired = await Processor(client, store, acl).PumpOnceAsync(5);
+
+        Assert.Equal(0, acquired);
+        Assert.Equal(1, client.AcquireCount);
+        Assert.Empty(client.Completed);
+    }
+}