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/Fakes.cs

@@ -4,8 +4,9 @@ using Big.Domain;
 namespace Big.Tests;
 
 /// <summary>An in-memory <see cref="IRegistrationStore"/> for the application-layer tests. Upserts
-/// keyed on the registration id, mirroring the production in-memory store.</summary>
-internal sealed class InMemoryRegistrationStore : IRegistrationStore
+/// keyed on the registration id, mirroring the production store (kept distinct from the production
+/// <c>InMemoryRegistrationStore</c> so tests can seed and inspect save counts).</summary>
+internal sealed class FakeRegistrationStore : IRegistrationStore
 {
     private readonly Dictionary<RegistrationId, Registration> _byId = [];