Improve testing for StreamableHttpHandler and IdleTrackingBackgroundService (#345)

* Add idle session pruning tests
* Remove redundant CTS from StreamableHttpHandler
  * StreamableHttpServerTransport.HandlePostRequest already creates the equivelant CTS
    which DeleteRequest_CompletesSession_WhichCancelsLongRunningToolCalls verifies
* Fix accept header validation for GET
* Use InvalidOperationException for transport-not-connected errors in doc comments
* Fix flaky Cancellation_ThrowsCancellationException test

tests/Common/Utils/TestServerTransport.cs

@@ -0,0 +1,84 @@
+using ModelContextProtocol.Protocol.Messages;
+using ModelContextProtocol.Protocol.Transport;
+using ModelContextProtocol.Protocol.Types;
+using ModelContextProtocol.Utils.Json;
+using System.Text.Json;
+using System.Threading.Channels;
+
+namespace ModelContextProtocol.Tests.Utils;
+
+public class TestServerTransport : ITransport
+{
+    private readonly Channel<JsonRpcMessage> _messageChannel;
+
+    public bool IsConnected { get; set; }
+
+    public ChannelReader<JsonRpcMessage> MessageReader => _messageChannel;
+
+    public List<JsonRpcMessage> SentMessages { get; } = [];
+
+    public Action<JsonRpcMessage>? OnMessageSent { get; set; }
+
+    public TestServerTransport()
+    {
+        _messageChannel = Channel.CreateUnbounded<JsonRpcMessage>(new UnboundedChannelOptions
+        {
+            SingleReader = true,
+            SingleWriter = true,
+        });
+        IsConnected = true;
+    }
+
+    public ValueTask DisposeAsync()
+    {
+        _messageChannel.Writer.TryComplete();
+        IsConnected = false;
+        return default;
+    }
+
+    public async Task SendMessageAsync(JsonRpcMessage message, CancellationToken cancellationToken = default)
+    {
+        SentMessages.Add(message);
+        if (message is JsonRpcRequest request)
+        {
+            if (request.Method == RequestMethods.RootsList)
+                await ListRoots(request, cancellationToken);
+            else if (request.Method == RequestMethods.SamplingCreateMessage)
+                await Sampling(request, cancellationToken);
+            else
+                await WriteMessageAsync(request, cancellationToken);
+        }
+        else if (message is JsonRpcNotification notification)
+        {
+            await WriteMessageAsync(notification, cancellationToken);
+        }
+
+        OnMessageSent?.Invoke(message);
+    }
+
+    private async Task ListRoots(JsonRpcRequest request, CancellationToken cancellationToken)
+    {
+        await WriteMessageAsync(new JsonRpcResponse
+        {
+            Id = request.Id,
+            Result = JsonSerializer.SerializeToNode(new ListRootsResult
+            {
+                Roots = []
+            }, McpJsonUtilities.DefaultOptions),
+        }, cancellationToken);
+    }
+
+    private async Task Sampling(JsonRpcRequest request, CancellationToken cancellationToken)
+    {
+        await WriteMessageAsync(new JsonRpcResponse
+        {
+            Id = request.Id,
+            Result = JsonSerializer.SerializeToNode(new CreateMessageResult { Content = new(), Model = "model", Role = Role.User }, McpJsonUtilities.DefaultOptions),
+        }, cancellationToken);
+    }
+
+    private async Task WriteMessageAsync(JsonRpcMessage message, CancellationToken cancellationToken = default)
+    {
+        await _messageChannel.Writer.WriteAsync(message, cancellationToken);
+    }
+}