feat: provide blanket implementations for ClientHandler and ServerHandler traits (#609)

* feat!: implement ServerHandler for Box<H> and Arc<H> where H is a ServerHandler

* feat!: implement ClientHandler for Box<H> and Arc<H> where H is a ClientHandler

* test: test Box and Arc have blanket implementations for handler traits

* refactor: deduplicate blanket implementations with macros

crates/rmcp/tests/test_handler_wrappers.rs

@@ -0,0 +1,28 @@
+// cargo test --test test_handler_wrappers --features "client server"
+
+mod common;
+
+use std::sync::Arc;
+
+use common::handlers::{TestClientHandler, TestServer};
+use rmcp::{ClientHandler, ServerHandler};
+
+#[test]
+fn test_wrapped_server_handlers() {
+    // This test asserts that, when T: ServerHandler, both Box<T> and Arc<T> also implement ServerHandler.
+    fn accepts_server_handler<H: ServerHandler>(_handler: H) {}
+
+    accepts_server_handler(Box::new(TestServer::new()));
+    accepts_server_handler(Arc::new(TestServer::new()));
+}
+
+#[test]
+fn test_wrapped_client_handlers() {
+    // This test asserts that, when T: ClientHandler, both Box<T> and Arc<T> also implement ClientHandler.
+    fn accepts_client_handler<H: ClientHandler>(_handler: H) {}
+
+    let client = TestClientHandler::new(false, false);
+
+    accepts_client_handler(Box::new(client.clone()));
+    accepts_client_handler(Arc::new(client));
+}