Fix GoalStream waiter resumption in action send_goal flow

[Timple]

I was trying this library on lyrical, noticed this was merged.

I'm running a single threaded executor:

rclcpp::executors::EventsCBGExecutor executor{rclcpp::ExecutorOptions{}, 1};

The change ensures the waiting GoalStream continuation is resumed reliably, so sequential send_goal calls continue without hanging.

[otamachan]

Thanks for the feedback!

The hang is real, but reverting to a direct resume() would reintroduce the Jazzy goal_requests_mutex deadlock (ros2/rclcpp#2796) that the post() was added to avoid(4c7937a). The root cause is that DrainWaitable didn't notify events-based executors. #54 fixes that hook instead, so both the deadlock fix and your EventsCBGExecutor case work. Could you check if it resolves the issue on your side?