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/SubmitRegistrationTests.cs

@@ -8,7 +8,7 @@ public class SubmitRegistrationTests
     [Fact]
     public async Task Submitting_persists_an_ingediend_registration_and_starts_the_process()
     {
-        var store = new InMemoryRegistrationStore();
+        var store = new FakeRegistrationStore();
         var workflow = new FakeWorkflowClient(processInstanceId: "proc-42");
         var handler = new SubmitRegistration(store, workflow);
 
@@ -28,7 +28,7 @@ public class SubmitRegistrationTests
     {
         // The worker correlates the OpenZaakAanmaken job back to the aggregate by id, so the
         // aggregate must already be persisted when the process starts (ADR-0009).
-        var store = new InMemoryRegistrationStore();
+        var store = new FakeRegistrationStore();
         Registration? visibleAtStart = null;
         var workflow = new FakeWorkflowClient(onStart: id => visibleAtStart = store.GetAsync(id).Result);
         var handler = new SubmitRegistration(store, workflow);
@@ -41,7 +41,7 @@ public class SubmitRegistrationTests
     [Fact]
     public async Task Submitting_without_a_bsn_is_rejected_and_starts_no_process()
     {
-        var store = new InMemoryRegistrationStore();
+        var store = new FakeRegistrationStore();
         var workflow = new FakeWorkflowClient();
         var handler = new SubmitRegistration(store, workflow);