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

src/ModelContextProtocol/Server/DestinationBoundMcpServer.cs

@@ -0,0 +1,37 @@
+using ModelContextProtocol.Protocol.Messages;
+using ModelContextProtocol.Protocol.Transport;
+using ModelContextProtocol.Protocol.Types;
+using System.Diagnostics;
+
+namespace ModelContextProtocol.Server;
+
+internal sealed class DestinationBoundMcpServer(McpServer server, ITransport? transport) : IMcpServer
+{
+    public string EndpointName => server.EndpointName;
+    public ClientCapabilities? ClientCapabilities => server.ClientCapabilities;
+    public Implementation? ClientInfo => server.ClientInfo;
+    public McpServerOptions ServerOptions => server.ServerOptions;
+    public IServiceProvider? Services => server.Services;
+    public LoggingLevel? LoggingLevel => server.LoggingLevel;
+
+    public ValueTask DisposeAsync() => server.DisposeAsync();
+
+    public IAsyncDisposable RegisterNotificationHandler(string method, Func<JsonRpcNotification, CancellationToken, ValueTask> handler) => server.RegisterNotificationHandler(method, handler);
+
+    // This will throw because the server must already be running for this class to be constructed, but it should give us a good Exception message.
+    public Task RunAsync(CancellationToken cancellationToken) => server.RunAsync(cancellationToken);
+
+    public Task SendMessageAsync(JsonRpcMessage message, CancellationToken cancellationToken = default)
+    {
+        Debug.Assert(message.RelatedTransport is null);
+        message.RelatedTransport = transport;
+        return server.SendMessageAsync(message, cancellationToken);
+    }
+
+    public Task<JsonRpcResponse> SendRequestAsync(JsonRpcRequest request, CancellationToken cancellationToken = default)
+    {
+        Debug.Assert(request.RelatedTransport is null);
+        request.RelatedTransport = transport;
+        return server.SendRequestAsync(request, cancellationToken);
+    }
+}