fix: gate client handler bounds for local (#1068)

crates/rmcp/tests/test_local_client_handler.rs

@@ -0,0 +1,16 @@
+use std::rc::Rc;
+
+use rmcp::ClientHandler;
+
+struct LocalClientHandler {
+    _state: Rc<()>,
+}
+
+impl ClientHandler for LocalClientHandler {}
+
+#[test]
+fn client_handler_accepts_non_send_sync_state_with_local_feature() {
+    fn assert_client_handler<T: ClientHandler>() {}
+
+    assert_client_handler::<LocalClientHandler>();
+}