fix: accept 200 with empty body in response to notifications in addition to 202 (#849)

crates/rmcp/tests/test_streamable_http_empty_2xx_notification.rs

@@ -0,0 +1,48 @@
+#![cfg(all(
+    feature = "transport-streamable-http-client",
+    feature = "transport-streamable-http-client-reqwest",
+    not(feature = "local")
+))]
+
+use std::{collections::HashMap, sync::Arc};
+
+use rmcp::{
+    model::{ClientJsonRpcMessage, ClientNotification, InitializedNotification},
+    transport::streamable_http_client::{StreamableHttpClient, StreamableHttpPostResponse},
+};
+
+async fn spawn_empty_ok_server() -> String {
+    use axum::{Router, http::StatusCode, routing::post};
+
+    let router = Router::new().route("/mcp", post(|| async { StatusCode::OK }));
+
+    let listener = tokio::net::TcpListener::bind("127.0.0.1:0").await.unwrap();
+    let addr = listener.local_addr().unwrap();
+    tokio::spawn(async move {
+        axum::serve(listener, router).await.unwrap();
+    });
+
+    format!("http://{addr}/mcp")
+}
+
+#[tokio::test]
+async fn empty_success_response_to_notification_is_accepted() {
+    let url = spawn_empty_ok_server().await;
+    let client = reqwest::Client::new();
+    let result = client
+        .post_message(
+            Arc::from(url.as_str()),
+            ClientJsonRpcMessage::notification(ClientNotification::InitializedNotification(
+                InitializedNotification::default(),
+            )),
+            None,
+            None,
+            HashMap::new(),
+        )
+        .await;
+
+    match result {
+        Ok(StreamableHttpPostResponse::Accepted) => {}
+        other => panic!("expected Accepted, got: {other:?}"),
+    }
+}