feat(S2-b): connect durable MQTT sync to capture workflow

tests/FieldLogger.Sync.Tests/FakeMqttTransport.cs

@@ -0,0 +1,61 @@
+using FieldLogger.Sync;
+
+namespace FieldLogger.Sync.Tests;
+
+/// <summary>
+/// In-memory stand-in for the broker. Records published payloads, lets a test toggle
+/// connectivity, force publish failures, and inject application-level acks — so the engine's
+/// durability/ack/backoff behaviour is exercised without MQTTnet or a live broker.
+/// </summary>
+public sealed class FakeMqttTransport : IMqttTransport
+{
+    public bool IsConnected { get; set; } = true;
+    public bool FailNextPublish { get; set; }
+    public List<(string Topic, byte[] Payload)> Published { get; } = new();
+
+    public event Func<AckBatch, Task>? AckReceived;
+
+    public Task ConnectAsync(string ackTopic, CancellationToken ct = default)
+    {
+        IsConnected = true;
+        return Task.CompletedTask;
+    }
+
+    public Task DisconnectAsync()
+    {
+        IsConnected = false;
+        return Task.CompletedTask;
+    }
+
+    public Task PublishAsync(string topic, byte[] payload, string schemaVersion, CancellationToken ct = default)
+    {
+        if (!IsConnected) throw new InvalidOperationException("not connected");
+        if (FailNextPublish)
+        {
+            FailNextPublish = false;
+            throw new InvalidOperationException("simulated broker publish failure");
+        }
+        Published.Add((topic, payload));
+        return Task.CompletedTask;
+    }
+
+    /// <summary>Simulate the cloud emitting an application-level ack for these pointIds.</summary>
+    public async Task InjectAckAsync(params AckItem[] items)
+    {
+        if (AckReceived is { } handlers)
+        {
+            var batch = new AckBatch { Results = items.ToList() };
+            foreach (Func<AckBatch, Task> handler in handlers.GetInvocationList())
+                await handler(batch);
+        }
+    }
+
+    public Task InjectAcceptAsync(string pointId) =>
+        InjectAckAsync(new AckItem { PointId = pointId, Outcome = AckOutcome.ACCEPTED });
+
+    public Task InjectDuplicateAsync(string pointId) =>
+        InjectAckAsync(new AckItem { PointId = pointId, Outcome = AckOutcome.DUPLICATE });
+
+    public Task InjectRejectAsync(string pointId, string reasonCode) =>
+        InjectAckAsync(new AckItem { PointId = pointId, Outcome = AckOutcome.REJECTED, ReasonCode = reasonCode });
+}