Add channel system unit and integration tests

Comprehensive NUnit test suite covering ChannelManager, CliChannel,
TelegramChannel, DI registration, and integration scenarios. Uses
Moq for mocking with test stubs for isolated unit testing.

Luna.Channels.Tests/Stubs/ChannelManagerStub.cs

@@ -0,0 +1,20 @@
+using Luna.Channels.Abstractions;
+using Luna.Core.Abstractions;
+
+namespace Luna.Channels.Tests.Stubs;
+
+public class ChannelManagerStub : IChannelManager
+{
+    public ChannelManagerStub() { }
+    public ChannelManagerStub(ISessionManager sessionManager) { }
+
+    public event EventHandler<ChannelMessageReceivedEventArgs>? MessageRouted;
+
+    public void RegisterChannel(IChannel channel) => throw new NotImplementedException();
+
+    public void UnregisterChannel(string channelId) => throw new NotImplementedException();
+
+    public IChannel? GetChannel(string channelId) => throw new NotImplementedException();
+
+    public IReadOnlyList<IChannel> GetAllChannels() => throw new NotImplementedException();
+}