fix: resolve E0063 and improve shutdown handling in JSON response path

- Add ..Default::default() to StreamableHttpServerConfig initializer in
  test_sse_concurrent_streams.rs to fix E0063 after json_response field
  was added (file introduced in upstream main after PR was branched)
- Expand tokio::select! arms so cancellation (server shutdown) returns
  HTTP 503 with tracing::info! instead of spurious HTTP 500 + error log

Made-with: Cursor

Author: thiagomendes

crates/rmcp/src/transport/streamable_http_server/tower.rs

@@ -603,27 +603,35 @@ where
                         // application/json, eliminating SSE framing overhead.
                         // Allowed by MCP Streamable HTTP spec (2025-06-18).
                         let cancel = self.config.cancellation_token.child_token();
-                        match tokio::select! {
-                            res = receiver.recv() => res,
-                            _ = cancel.cancelled() => None,
-                        } {
-                            Some(message) => {
-                                tracing::info!(?message);
-                                let body = serde_json::to_vec(&message).map_err(|e| {
-                                    internal_error_response("serialize json response")(e)
-                                })?;
-                                Ok(Response::builder()
-                                    .status(http::StatusCode::OK)
-                                    .header(http::header::CONTENT_TYPE, JSON_MIME_TYPE)
-                                    .body(Full::new(Bytes::from(body)).boxed())
+                        tokio::select! {
+                            res = receiver.recv() => match res {
+                                Some(message) => {
+                                    tracing::info!(?message);
+                                    let body = serde_json::to_vec(&message).map_err(|e| {
+                                        internal_error_response("serialize json response")(e)
+                                    })?;
+                                    Ok(Response::builder()
+                                        .status(http::StatusCode::OK)
+                                        .header(http::header::CONTENT_TYPE, JSON_MIME_TYPE)
+                                        .body(Full::new(Bytes::from(body)).boxed())
+                                        .expect("valid response"))
+                                }
+                                None => Err(internal_error_response("empty response")(
+                                    std::io::Error::new(
+                                        std::io::ErrorKind::UnexpectedEof,
+                                        "no response message received from handler",
+                                    ),
+                                )),
+                            },
+                            _ = cancel.cancelled() => {
+                                tracing::info!("JSON response cancelled due to server shutdown");
+                                Err(Response::builder()
+                                    .status(http::StatusCode::SERVICE_UNAVAILABLE)
+                                    .body(
+                                        Full::new(Bytes::from("Server is shutting down")).boxed(),
+                                    )
                                     .expect("valid response"))
                             }
-                            None => Err(internal_error_response("empty response")(
-                                std::io::Error::new(
-                                    std::io::ErrorKind::UnexpectedEof,
-                                    "no response message received from handler",
-                                ),
-                            )),
                         }
                     } else {
                         // SSE mode (default): original behaviour preserved unchanged

crates/rmcp/tests/test_sse_concurrent_streams.rs

@@ -84,6 +84,7 @@ async fn start_test_server(ct: CancellationToken, trigger: Arc<Notify>) -> Strin
             sse_keep_alive: Some(Duration::from_secs(15)),
             sse_retry: Some(Duration::from_secs(3)),
             cancellation_token: ct.child_token(),
+            ..Default::default()
         },
     );