mission_behavior_tree.py doesn't start the behavior reliably

When running mission_behavior_tree.py, I run into the issue that the actual behavior won't reliably start. In order to fix this, I modified the script like this

```
import rclpy
from rclpy.node import Node
from rclpy.qos import QoSProfile, ReliabilityPolicy, HistoryPolicy
from std_msgs.msg import String
import time

class StartBehaviorTree(Node):
"""Drone interface base node"""
def __init__(self, namespace):
super().__init__('start_bt', namespace=namespace)

# Create a reliable QoS profile
qos_profile = QoSProfile(
reliability=ReliabilityPolicy.RELIABLE,
history=HistoryPolicy.KEEP_ALL,
depth=10
)

self.start_pub = self.create_publisher(
String, "start", qos_profile)

# Wait for publisher to be ready
while self.start_pub.get_subscription_count() == 0:
self.get_logger().info('Waiting for subscribers...')
time.sleep(0.1)

# Publish message
msg = String()
msg.data = "Start behavior tree"
self.start_pub.publish(msg)
self.get_logger().info('Published start message')

# Ensure message is sent before shutting down
time.sleep(0.5)

# Initiate shutdown
self.get_logger().info('Initiating node shutdown')
self.destroy_node()

if __name__ == "__main__":
rclpy.init()
start_bt = StartBehaviorTree(namespace="drone0")
rclpy.spin_once(start_bt)
rclpy.shutdown()
```

This makes the behavior launch every time. The one downside is that this node won't shutdown properly.

It seems that the publisher's message to /drone0/start in the original script isn't broadcasting properly. This might be due to the node shutting down too fast, or because of a lack of connection to subscribers. Whenever I publish manually using the ROS2 CLI, the behavior always launches, so I'm assuming there is some kind of timing issue going on.