fix feature flags

Author: Vaiz

crates/rust-mcp-sdk/src/mcp_traits.rs

@@ -1,5 +1,6 @@
 #[cfg(feature = "client")]
 pub mod mcp_client;
+#[cfg(any(feature = "server", feature = "client"))]
 pub mod mcp_handler;
 #[cfg(feature = "server")]
 pub mod mcp_server;