Add ActiveDevice + DeviceRegistry + HubEnvelopes + IDeviceClient/Channel implementations

backend.tests/DeviceRegistryTests.cs

@@ -0,0 +1,25 @@
+using System.Net.WebSockets;
+using backend.DeviceHub;
+using FluentAssertions;
+using Xunit;
+
+namespace backend.tests;
+
+public class DeviceRegistryTests
+{
+    [Fact]
+    public void Register_unregister_round_trip()
+    {
+        var reg = new DeviceRegistry();
+        var id = Guid.NewGuid();
+        var dev = new ActiveDevice(id, new ClientWebSocket());
+        reg.TryRegister(dev).Should().BeTrue();
+        reg.TryRegister(dev).Should().BeFalse();
+        reg.IsOnline(id).Should().BeTrue();
+        reg.OnlineCount.Should().Be(1);
+
+        reg.Unregister(id).Should().BeTrue();
+        reg.IsOnline(id).Should().BeFalse();
+        reg.OnlineCount.Should().Be(0);
+    }
+}

backend.tests/FakeDeviceChannel.cs

@@ -1,14 +1,28 @@
 using System.Collections.Concurrent;
 using System.Text.Json;
+using backend.DeviceHub;
 using backend.Tools;
 
 namespace backend.tests;
 
-// Slim version for Tasks 9-15. Task 16 reopens this file to add IDeviceClient + envelope sending.
-public class FakeDeviceChannel : IDeviceChannel
+public class FakeDeviceChannel : IDeviceChannel, IDeviceClient
 {
+    public List<HubEnvelope> SentEnvelopes { get; } = new();
+    public List<byte[]> SentBinary { get; } = new();
     public ConcurrentQueue<JsonElement> PiToolReplies { get; } = new();
 
+    public Task SendEnvelopeAsync<T>(T envelope, CancellationToken ct) where T : HubEnvelope
+    {
+        SentEnvelopes.Add(envelope);
+        return Task.CompletedTask;
+    }
+
+    public Task SendBinaryAsync(ReadOnlyMemory<byte> bytes, CancellationToken ct)
+    {
+        SentBinary.Add(bytes.ToArray());
+        return Task.CompletedTask;
+    }
+
     public Task<JsonElement> CallPiToolAsync(string name, JsonElement args, CancellationToken ct)
     {
         if (PiToolReplies.TryDequeue(out var reply))