feat: add actix-web support for streamable HTTP server transport

- Implement ActixStreamableHttpService with full feature parity to Axum implementation
- Support GET (SSE streams), POST (requests), DELETE (session cleanup) endpoints
- Add comprehensive logging matching SSE implementation
- Use X-Accel-Buffering header constant for consistency
- Create working example server (counter_streamable_http_actix.rs)
- Refactor tower.rs to import StreamableHttpServerConfig from parent module
- Add HEADER_X_ACCEL_BUFFERING constant to common http_header module
- Update streamable_client.js to accept URL as command line argument
- Configure example to bind to 127.0.0.1 for IPv4 compatibility

The actix-web implementation provides identical functionality to the Axum version
while following actix-web patterns and conventions.

Author: JMLX42

crates/rmcp/src/transport/common/http_header.rs

@@ -1,4 +1,5 @@
 pub const HEADER_SESSION_ID: &str = "Mcp-Session-Id";
 pub const HEADER_LAST_EVENT_ID: &str = "Last-Event-Id";
+pub const HEADER_X_ACCEL_BUFFERING: &str = "X-Accel-Buffering";
 pub const EVENT_STREAM_MIME_TYPE: &str = "text/event-stream";
 pub const JSON_MIME_TYPE: &str = "application/json";

crates/rmcp/src/transport/streamable_http_server.rs

@@ -1,8 +1,76 @@
+//! Streamable HTTP Server Transport Module
+//!
+//! This module provides streamable HTTP transport implementations for MCP.
+//!
+//! # Type Export Strategy
+//!
+//! This module exports framework-specific implementations with explicit names:
+//! - `AxumStreamableHttpService` - The Axum-based streamable HTTP service implementation
+//! - `ActixStreamableHttpService` - The actix-web-based streamable HTTP service implementation
+//!
+//! For convenience, a type alias `StreamableHttpService` is provided that resolves to:
+//! - `ActixStreamableHttpService` when the `actix-web` feature is enabled
+//! - `AxumStreamableHttpService` when only the `axum` feature is enabled
+//!
+//! # Examples
+//!
+//! Using the convenience alias (recommended for most use cases):
+//! ```ignore
+//! use rmcp::transport::StreamableHttpService;
+//! let service = StreamableHttpService::new(|| Ok(handler), session_manager, config);
+//! ```
+//!
+//! Using explicit types (when you need a specific implementation):
+//! ```ignore
+//! #[cfg(feature = "axum")]
+//! use rmcp::transport::AxumStreamableHttpService;
+//! #[cfg(feature = "axum")]
+//! let service = AxumStreamableHttpService::new(|| Ok(handler), session_manager, config);
+//! ```
+
 pub mod session;
-#[cfg(feature = "transport-streamable-http-server")]
-#[cfg_attr(docsrs, doc(cfg(feature = "transport-streamable-http-server")))]
+
+use std::time::Duration;
+
+/// Configuration for the streamable HTTP server
+#[derive(Debug, Clone)]
+pub struct StreamableHttpServerConfig {
+    /// The ping message duration for SSE connections.
+    pub sse_keep_alive: Option<Duration>,
+    /// If true, the server will create a session for each request and keep it alive.
+    pub stateful_mode: bool,
+}
+
+impl Default for StreamableHttpServerConfig {
+    fn default() -> Self {
+        Self {
+            sse_keep_alive: Some(Duration::from_secs(15)),
+            stateful_mode: true,
+        }
+    }
+}
+
+// Axum implementation
+#[cfg(all(feature = "transport-streamable-http-server", feature = "axum"))]
+#[cfg_attr(docsrs, doc(cfg(all(feature = "transport-streamable-http-server", feature = "axum"))))]
 pub mod tower;
+
+#[cfg(all(feature = "transport-streamable-http-server", feature = "axum"))]
+pub use tower::StreamableHttpService as AxumStreamableHttpService;
+
+// Actix-web implementation
+#[cfg(all(feature = "transport-streamable-http-server", feature = "actix-web"))]
+#[cfg_attr(docsrs, doc(cfg(all(feature = "transport-streamable-http-server", feature = "actix-web"))))]
+pub mod actix_impl;
+
+#[cfg(all(feature = "transport-streamable-http-server", feature = "actix-web"))]
+pub use actix_impl::StreamableHttpService as ActixStreamableHttpService;
+
+// Export the preferred implementation as StreamableHttpService (without generic parameters)
+#[cfg(all(feature = "transport-streamable-http-server", feature = "actix-web"))]
+pub use actix_impl::StreamableHttpService;
+
+#[cfg(all(feature = "transport-streamable-http-server", feature = "axum", not(feature = "actix-web")))]
+pub use tower::StreamableHttpService;
+
 pub use session::{SessionId, SessionManager};
-#[cfg(feature = "transport-streamable-http-server")]
-#[cfg_attr(docsrs, doc(cfg(feature = "transport-streamable-http-server")))]
-pub use tower::{StreamableHttpServerConfig, StreamableHttpService};