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.AspNetCore.Tests/Utils/KestrelInMemoryTest.cs

@@ -27,7 +27,11 @@ public class KestrelInMemoryTest : LoggedTest
                 var connection = _inMemoryTransport.CreateConnection();
                 return new(connection.ClientStream);
             },
-        });
+        })
+        {
+            BaseAddress = new Uri("http://localhost:5000/"),
+            Timeout = TimeSpan.FromSeconds(10),
+        };
     }
 
     public WebApplicationBuilder Builder { get; }

tests/ModelContextProtocol.AspNetCore.Tests/Utils/TestServerTransport.cs

@@ -8,19 +8,19 @@ 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,
@@ -35,7 +35,7 @@ public class TestServerTransport : ITransport
         return ValueTask.CompletedTask;
     }
 
-    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)
@@ -76,7 +76,7 @@ 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);
     }