Add server-side Streamable HTTP transport support (#330)

- Changes IJsonRpcMessage to an abstract base class so RelatedTransport will always be available
- Streamable HTTP supports multiple concurrent HTTP request with their own indpendent SSE response streams
- RelatedTransport indicates the source or destination of the JsonRpcMessage

- Changes the default RequestId to a JSON number for better compatibility with MCP servers in the wild

tests/ModelContextProtocol.Tests/Client/McpClientFactoryTests.cs

@@ -106,11 +106,11 @@ public class McpClientFactoryTests
 
     private class NopTransport : ITransport, IClientTransport
     {
-        private readonly Channel<IJsonRpcMessage> _channel = Channel.CreateUnbounded<IJsonRpcMessage>();
+        private readonly Channel<JsonRpcMessage> _channel = Channel.CreateUnbounded<JsonRpcMessage>();
 
         public bool IsConnected => true;
 
-        public ChannelReader<IJsonRpcMessage> MessageReader => _channel.Reader;
+        public ChannelReader<JsonRpcMessage> MessageReader => _channel.Reader;
 
         public Task<ITransport> ConnectAsync(CancellationToken cancellationToken = default) => Task.FromResult<ITransport>(this);
 
@@ -118,7 +118,7 @@ public class McpClientFactoryTests
 
         public string Name => "Test Nop Transport";
 
-        public virtual Task SendMessageAsync(IJsonRpcMessage message, CancellationToken cancellationToken = default)
+        public virtual Task SendMessageAsync(JsonRpcMessage message, CancellationToken cancellationToken = default)
         {
             switch (message)
             {
@@ -148,7 +148,7 @@ public class McpClientFactoryTests
     {
         public const string ExpectedMessage = "Something failed";
 
-        public override Task SendMessageAsync(IJsonRpcMessage message, CancellationToken cancellationToken = default)
+        public override Task SendMessageAsync(JsonRpcMessage message, CancellationToken cancellationToken = default)
         {
             throw new InvalidOperationException(ExpectedMessage);
         }

tests/ModelContextProtocol.Tests/Protocol/NotificationHandlerTests.cs

@@ -33,7 +33,7 @@ public class NotificationHandlerTests : ClientServerTestBase
             }
         }
 
-        Assert.Equal(Iterations, counter);  
+        Assert.Equal(Iterations, counter);
     }
 
     [Fact]

tests/ModelContextProtocol.Tests/Server/McpServerTests.cs

@@ -619,7 +619,7 @@ public class McpServerTests : LoggedTest
         public McpServerOptions ServerOptions => throw new NotImplementedException();
         public IServiceProvider? Services => throw new NotImplementedException();
         public LoggingLevel? LoggingLevel => throw new NotImplementedException();
-        public Task SendMessageAsync(IJsonRpcMessage message, CancellationToken cancellationToken = default) =>
+        public Task SendMessageAsync(JsonRpcMessage message, CancellationToken cancellationToken = default) =>
             throw new NotImplementedException();
         public Task RunAsync(CancellationToken cancellationToken = default) =>
             throw new NotImplementedException();

tests/ModelContextProtocol.Tests/Transport/SseClientTransportTests.cs

@@ -216,6 +216,7 @@ public class SseClientTransportTests : LoggedTest
 
         await session.DisposeAsync();
 
-        Assert.False(session.IsConnected);
+        var transportBase = Assert.IsAssignableFrom<TransportBase>(session);
+        Assert.False(transportBase.IsConnected);
     }
 }

tests/ModelContextProtocol.Tests/Utils/TestServerTransport.cs

@@ -9,19 +9,19 @@ namespace ModelContextProtocol.Tests.Utils;
 
 public class TestServerTransport : ITransport
 {
-    private readonly Channel<IJsonRpcMessage> _messageChannel;
+    private readonly Channel<JsonRpcMessage> _messageChannel;
 
     public bool IsConnected { get; set; }
 
-    public ChannelReader<IJsonRpcMessage> MessageReader => _messageChannel;
+    public ChannelReader<JsonRpcMessage> MessageReader => _messageChannel;
 
-    public List<IJsonRpcMessage> SentMessages { get; } = [];
+    public List<JsonRpcMessage> SentMessages { get; } = [];
 
-    public Action<IJsonRpcMessage>? OnMessageSent { get; set; }
+    public Action<JsonRpcMessage>? OnMessageSent { get; set; }
 
     public TestServerTransport()
     {
-        _messageChannel = Channel.CreateUnbounded<IJsonRpcMessage>(new UnboundedChannelOptions
+        _messageChannel = Channel.CreateUnbounded<JsonRpcMessage>(new UnboundedChannelOptions
         {
             SingleReader = true,
             SingleWriter = true,
@@ -36,7 +36,7 @@ public class TestServerTransport : ITransport
         return default;
     }
 
-    public async Task SendMessageAsync(IJsonRpcMessage message, CancellationToken cancellationToken = default)
+    public async Task SendMessageAsync(JsonRpcMessage message, CancellationToken cancellationToken = default)
     {
         SentMessages.Add(message);
         if (message is JsonRpcRequest request)
@@ -77,7 +77,7 @@ public class TestServerTransport : ITransport
         }, cancellationToken);
     }
 
-    private async Task WriteMessageAsync(IJsonRpcMessage message, CancellationToken cancellationToken = default)
+    private async Task WriteMessageAsync(JsonRpcMessage message, CancellationToken cancellationToken = default)
     {
         await _messageChannel.Writer.WriteAsync(message, cancellationToken);
     }