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/AclHttpClient.cs

@@ -0,0 +1,17 @@
+using Big.Application;
+
+namespace Big.Infrastructure;
+
+/// <summary>
+/// HTTP client to the ACL service — the boundary the domain crosses to open a zaak (§8.1). It POSTs
+/// the bsn to the ACL's <c>/zaken</c> endpoint and returns the created zaak URL; it never constructs
+/// ZGW URLs or talks to OpenZaak itself.
+/// </summary>
+public sealed class AclHttpClient(HttpClient http, AclOptions options) : IAclClient
+{
+    public Task<Uri> OpenZaakAsync(string bsn, CancellationToken ct = default)
+    {
+        // STUB (red).
+        return Task.FromResult(new Uri("about:blank"));
+    }
+}

services/domain/Big.Infrastructure/Big.Infrastructure.csproj

@@ -0,0 +1,24 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <!-- The infrastructure layer (CLAUDE.md §9): adapters implementing the Application ports.
+       The Workflow Client (Flowable REST) and the ACL HTTP client live here — the only code
+       that talks to Flowable (§8.2) and to the ACL respectively. -->
+  <PropertyGroup>
+    <TargetFramework>net10.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\Big.Application\Big.Application.csproj" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <!-- The external-task job worker is a hosted BackgroundService (ADR-0009); it logs and
+         resolves a per-tick scope. Abstractions only — the host (Api) brings the implementations. -->
+    <PackageReference Include="Microsoft.Extensions.Hosting.Abstractions" Version="10.0.0" />
+    <PackageReference Include="Microsoft.Extensions.Logging.Abstractions" Version="10.0.0" />
+    <PackageReference Include="Microsoft.Extensions.DependencyInjection.Abstractions" Version="10.0.0" />
+  </ItemGroup>
+
+</Project>

services/domain/Big.Infrastructure/FlowableWorkflowClient.cs

@@ -0,0 +1,31 @@
+using Big.Application;
+using Big.Domain;
+
+namespace Big.Infrastructure;
+
+/// <summary>
+/// The Workflow Client — the only code that talks to Flowable (§8.2). It starts registratie process
+/// instances and drives the <c>OpenZaakAanmaken</c> external-worker jobs over Flowable's REST API
+/// (start: <c>service/runtime/process-instances</c>; acquire/complete: <c>external-job-api/…</c>).
+/// </summary>
+public sealed class FlowableWorkflowClient(HttpClient http, FlowableOptions options)
+    : IWorkflowClient, IExternalWorkerClient
+{
+    public Task<string> StartRegistrationProcessAsync(RegistrationId registrationId, CancellationToken ct = default)
+    {
+        // STUB (red).
+        return Task.FromResult("");
+    }
+
+    public Task<IReadOnlyList<OpenZaakJob>> AcquireOpenZaakJobsAsync(int maxJobs, CancellationToken ct = default)
+    {
+        // STUB (red).
+        return Task.FromResult<IReadOnlyList<OpenZaakJob>>([]);
+    }
+
+    public Task CompleteOpenZaakJobAsync(string jobId, Uri zaakUrl, CancellationToken ct = default)
+    {
+        // STUB (red).
+        return Task.CompletedTask;
+    }
+}

services/domain/Big.Infrastructure/IExternalWorkerClient.cs

@@ -0,0 +1,18 @@
+using Big.Application;
+
+namespace Big.Infrastructure;
+
+/// <summary>
+/// The worker-facing side of the Workflow Client: acquiring and completing Flowable external-worker
+/// jobs (ADR-0009). Kept separate from the Application's <see cref="IWorkflowClient"/> because job
+/// acquisition is a Flowable-specific polling mechanic the application never needs to know about.
+/// Implemented by <see cref="FlowableWorkflowClient"/> — the only code that talks to Flowable (§8.2).
+/// </summary>
+public interface IExternalWorkerClient
+{
+    /// <summary>Acquire and lock up to <paramref name="maxJobs"/> <c>OpenZaakAanmaken</c> jobs.</summary>
+    Task<IReadOnlyList<OpenZaakJob>> AcquireOpenZaakJobsAsync(int maxJobs, CancellationToken ct = default);
+
+    /// <summary>Complete an acquired job, passing the opened zaak URL back into the process.</summary>
+    Task CompleteOpenZaakJobAsync(string jobId, Uri zaakUrl, CancellationToken ct = default);
+}

services/domain/Big.Infrastructure/InMemoryRegistrationStore.cs

@@ -0,0 +1,27 @@
+using System.Collections.Concurrent;
+using Big.Application;
+using Big.Domain;
+
+namespace Big.Infrastructure;
+
+/// <summary>
+/// In-memory <see cref="IRegistrationStore"/> for the minimal slice (ADR-0009). The walking
+/// skeleton's read path is the projection (S-06), not this store, so durable domain persistence is a
+/// documented follow-up. Registered as a singleton so the submit endpoint and the worker share it.
+/// </summary>
+public sealed class InMemoryRegistrationStore : IRegistrationStore
+{
+    private readonly ConcurrentDictionary<RegistrationId, Registration> _byId = new();
+
+    public Task SaveAsync(Registration registration, CancellationToken ct = default)
+    {
+        // STUB (red).
+        return Task.CompletedTask;
+    }
+
+    public Task<Registration?> GetAsync(RegistrationId id, CancellationToken ct = default)
+    {
+        // STUB (red).
+        return Task.FromResult<Registration?>(null);
+    }
+}

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;
+    }
+}

services/domain/Big.Infrastructure/Options.cs

@@ -0,0 +1,29 @@
+namespace Big.Infrastructure;
+
+/// <summary>Configuration for the Flowable Workflow Client. <see cref="BaseUrl"/> is the flowable-rest
+/// root and must end with a slash (e.g. <c>http://flowable-rest:8080/flowable-rest/</c>) so the
+/// <c>service/…</c> and <c>external-job-api/…</c> sub-paths resolve correctly.</summary>
+public sealed class FlowableOptions
+{
+    public Uri BaseUrl { get; set; } = null!;
+    public string Username { get; set; } = "";
+    public string Password { get; set; } = "";
+
+    /// <summary>The id this worker locks jobs under, so two workers don't process the same job.</summary>
+    public string WorkerId { get; set; } = "big-domain-worker";
+
+    /// <summary>How long an acquired job stays locked to this worker (ISO-8601 duration).</summary>
+    public string LockDuration { get; set; } = "PT5M";
+
+    /// <summary>How many OpenZaakAanmaken jobs to acquire per poll.</summary>
+    public int MaxJobsPerPoll { get; set; } = 5;
+
+    /// <summary>How often the worker polls Flowable for new jobs.</summary>
+    public TimeSpan PollInterval { get; set; } = TimeSpan.FromSeconds(2);
+}
+
+/// <summary>Configuration for the ACL HTTP client. <see cref="BaseUrl"/> is the ACL service root.</summary>
+public sealed class AclOptions
+{
+    public Uri BaseUrl { get; set; } = null!;
+}