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/OpenZaakWorkerTests.cs

@@ -16,7 +16,7 @@ public class OpenZaakWorkerTests
     public async Task Handling_a_job_opens_a_zaak_via_the_acl_and_attaches_it()
     {
         var registration = Submitted();
-        var store = new InMemoryRegistrationStore();
+        var store = new FakeRegistrationStore();
         store.Seed(registration);
         var acl = new FakeAclClient();
         var worker = new OpenZaakWorker(store, acl);
@@ -32,7 +32,7 @@ public class OpenZaakWorkerTests
     [Fact]
     public async Task Handling_a_job_for_an_unknown_registration_throws_and_opens_no_zaak()
     {
-        var store = new InMemoryRegistrationStore();
+        var store = new FakeRegistrationStore();
         var acl = new FakeAclClient();
         var worker = new OpenZaakWorker(store, acl);
 
@@ -45,7 +45,7 @@ public class OpenZaakWorkerTests
     public async Task Handling_a_redelivered_job_is_idempotent_and_does_not_open_a_second_zaak()
     {
         var registration = Submitted();
-        var store = new InMemoryRegistrationStore();
+        var store = new FakeRegistrationStore();
         store.Seed(registration);
         var acl = new FakeAclClient();
         var worker = new OpenZaakWorker(store, acl);