Fix and enhance cancellation operations across MCP Sessions. (#179)

* Propagate CancellationToken request cancellation to remote endpoint

* Refactor existing tests to have a shared base class

* Add cancellation tests

---------

Co-authored-by: Stephen Toub <stoub@microsoft.com>

tests/ModelContextProtocol.Tests/Protocol/CancellationTests.cs

@@ -0,0 +1,69 @@
+using Microsoft.Extensions.DependencyInjection;
+using ModelContextProtocol.Client;
+using ModelContextProtocol.Protocol.Messages;
+using ModelContextProtocol.Server;
+
+namespace ModelContextProtocol.Tests;
+
+public class CancellationTests : ClientServerTestBase
+{
+    public CancellationTests(ITestOutputHelper testOutputHelper)
+        : base(testOutputHelper)
+    {
+    }
+
+    protected override void ConfigureServices(ServiceCollection services, IMcpServerBuilder mcpServerBuilder)
+    {
+        services.AddSingleton(McpServerTool.Create(WaitForCancellation));
+    }
+
+    private static async Task WaitForCancellation(CancellationToken cancellationToken)
+    {
+        try
+        {
+            await Task.Delay(-1, cancellationToken);
+            throw new InvalidOperationException("Unexpected completion without exception");
+        }
+        catch (OperationCanceledException)
+        {
+            return;
+        }
+    }
+
+    [Fact]
+    public async Task PrecancelRequest_CancelsBeforeSending()
+    {
+        var client = await CreateMcpClientForServer();
+
+        bool gotCancellation = false;
+        await using (Server.RegisterNotificationHandler(NotificationMethods.CancelledNotification, (notification, cancellationToken) =>
+        {
+            gotCancellation = true;
+            return Task.CompletedTask;
+        }))
+        {
+            await Assert.ThrowsAsync<OperationCanceledException>(() => client.ListToolsAsync(cancellationToken: new CancellationToken(true)));
+        }
+
+        Assert.False(gotCancellation);
+    }
+
+    [Fact]
+    public async Task CancellationPropagation_RequestingCancellationCancelsPendingRequest()
+    {
+        var client = await CreateMcpClientForServer();
+
+        var tools = await client.ListToolsAsync(cancellationToken: TestContext.Current.CancellationToken);
+        var waitTool = tools.First(t => t.Name == nameof(WaitForCancellation));
+
+        CancellationTokenSource cts = new();
+        var waitTask = waitTool.InvokeAsync(cancellationToken: cts.Token);
+        Assert.False(waitTask.IsCompleted);
+
+        await Task.Delay(1, TestContext.Current.CancellationToken);
+        Assert.False(waitTask.IsCompleted);
+
+        cts.Cancel();
+        await Assert.ThrowsAnyAsync<OperationCanceledException>(async () => await waitTask);
+    }
+}