Add an axum sse example (#12)

* add axum sse example

* fix: make the sse axum folder name correct

* feat: add common modules for Axum SSE example

This commit adds supporting modules for the Axum Server-Sent Events (SSE) example:
- Added `counter.rs` with a simple counter router implementation
- Added `jsonrpc_frame_codec.rs` for decoding JSON-RPC frames
- Created a `mod.rs` to expose these modules
- Removed the previous SSE example configuration from Cargo.toml

* move examples to root

* nit: remove extra newline here

crates/mcp-server/src/lib.rs

@@ -142,7 +142,8 @@ where
 
         tracing::info!("Server started");
         while let Some(msg_result) = transport.next().await {
-            let _span = tracing::span!(tracing::Level::INFO, "message_processing").entered();
+            let _span = tracing::span!(tracing::Level::INFO, "message_processing");
+            let _enter = _span.enter();
             match msg_result {
                 Ok(msg) => {
                     match msg {