fix(gateway): switch to MultiThreadedExecutor for thread-safe subscription creation

SingleThreadedExecutor does not synchronize Node::create_generic_subscription()
called from external threads (cpp-httplib handlers) with its internal wait set
iteration. This causes non-deterministic SIGSEGV on rolling where rclcpp
internals are less forgiving of concurrent node manipulation.

MultiThreadedExecutor properly synchronizes subscription lifecycle across
threads. All gateway callbacks are already protected by mutexes (EntityCache,
LogManager, TriggerManager, TriggerTopicSubscriber), so concurrent callback
execution is safe.

This is the definitive fix for the rolling SIGSEGV in test_data_read.
The previous sampling_mutex_ + MutuallyExclusiveCallbackGroup fix reduced
the race window but did not eliminate it.

Author: bburda

src/ros2_medkit_gateway/src/main.cpp

@@ -21,7 +21,14 @@ int main(int argc, char ** argv) {
 
   auto node = std::make_shared<ros2_medkit_gateway::GatewayNode>();
 
-  rclcpp::spin(node);
+  // MultiThreadedExecutor is required because cpp-httplib handler threads call
+  // Node::create_generic_subscription() for topic sampling. SingleThreadedExecutor
+  // does not synchronize external subscription creation with its internal iteration,
+  // causing non-deterministic SIGSEGV on rolling. All gateway callbacks are already
+  // protected by mutexes (EntityCache, LogManager, TriggerManager, etc.).
+  rclcpp::executors::MultiThreadedExecutor executor;
+  executor.add_node(node);
+  executor.spin();
 
   rclcpp::shutdown();
   return 0;