Flow ExecutionContext with JsonRpcMessage (#616)

The primary goal of this change is to support IHttpContextAccessor in tool calls when the Streamable HTTP is in its default non-Stateless mode.

src/ModelContextProtocol.Core/Server/StreamableHttpPostTransport.cs

@@ -91,6 +91,11 @@ internal sealed class StreamableHttpPostTransport(StreamableHttpServerTransport
 
         message.RelatedTransport = this;
 
+        if (parentTransport.FlowExecutionContextFromRequests)
+        {
+            message.ExecutionContext = ExecutionContext.Capture();
+        }
+
         await parentTransport.MessageWriter.WriteAsync(message, cancellationToken).ConfigureAwait(false);
     }
 }

src/ModelContextProtocol.Core/Server/StreamableHttpServerTransport.cs

@@ -10,7 +10,7 @@ namespace ModelContextProtocol.Server;
 /// <remarks>
 /// <para>
 /// This transport provides one-way communication from server to client using the SSE protocol over HTTP,
-/// while receiving client messages through a separate mechanism. It writes messages as 
+/// while receiving client messages through a separate mechanism. It writes messages as
 /// SSE events to a response stream, typically associated with an HTTP response.
 /// </para>
 /// <para>
@@ -36,6 +36,9 @@ public sealed class StreamableHttpServerTransport : ITransport
 
     private int _getRequestStarted;
 
+    /// <inheritdoc/>
+    public string? SessionId { get; set; }
+
     /// <summary>
     /// Configures whether the transport should be in stateless mode that does not require all requests for a given session
     /// to arrive to the same ASP.NET Core application process. Unsolicited server-to-client messages are not supported in this mode,
@@ -45,6 +48,15 @@ public sealed class StreamableHttpServerTransport : ITransport
     /// </summary>
     public bool Stateless { get; init; }
 
+    /// <summary>
+    /// Gets a value indicating whether the execution context should flow from the calls to <see cref="HandlePostRequest(IDuplexPipe, CancellationToken)"/>
+    /// to the corresponding <see cref="JsonRpcMessage.ExecutionContext"/> emitted by the <see cref="MessageReader"/>.
+    /// </summary>
+    /// <remarks>
+    /// Defaults to <see langword="false"/>.
+    /// </remarks>
+    public bool FlowExecutionContextFromRequests { get; init; }
+
     /// <summary>
     /// Gets or sets a callback to be invoked before handling the initialize request.
     /// </summary>
@@ -55,9 +67,6 @@ public sealed class StreamableHttpServerTransport : ITransport
 
     internal ChannelWriter<JsonRpcMessage> MessageWriter => _incomingChannel.Writer;
 
-    /// <inheritdoc/>
-    public string? SessionId { get; set; }
-
     /// <summary>
     /// Handles an optional SSE GET request a client using the Streamable HTTP transport might make by
     /// writing any unsolicited JSON-RPC messages sent via <see cref="SendMessageAsync"/>