chore: import upstream snapshot with attribution

dotnet/tests/Microsoft.Agents.AI.Hosting.A2A.UnitTests/Internal/DummyChatClient.cs

@@ -0,0 +1,30 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+using System;
+using System.Collections.Generic;
+using System.Threading;
+using System.Threading.Tasks;
+using Microsoft.Extensions.AI;
+
+namespace Microsoft.Agents.AI.Hosting.A2A.UnitTests.Internal;
+
+internal sealed class DummyChatClient : IChatClient
+{
+    public void Dispose()
+    {
+        throw new NotImplementedException();
+    }
+
+    public Task<ChatResponse> GetResponseAsync(IEnumerable<ChatMessage> messages, ChatOptions? options = null, CancellationToken cancellationToken = default)
+    {
+        throw new NotImplementedException();
+    }
+
+    public object? GetService(Type serviceType, object? serviceKey = null) =>
+        serviceType.IsInstanceOfType(this) ? this : null;
+
+    public IAsyncEnumerable<ChatResponseUpdate> GetStreamingResponseAsync(IEnumerable<ChatMessage> messages, ChatOptions? options = null, CancellationToken cancellationToken = default)
+    {
+        throw new NotImplementedException();
+    }
+}