Fix race condition in ActionBasedTreeExecutorNode: per-goal TreeConstructor storage via UUID map

[Copilot]

pending_tree_constructor_ was a single shared member written in handle_trigger_goal_() and consumed in handle_trigger_accept_(). Two goals accepted before startExecution() marked the executor busy would overwrite each other's constructor, causing the wrong tree to execute.

Changes

• pending_tree_constructor_ → pending_tree_constructors_: replaced the single TreeConstructor with std::map<rclcpp_action::GoalUUID, TreeConstructor>, keyed by goal UUID.
• handle_trigger_goal_(): stores the constructor into the map under the incoming UUID instead of overwriting a shared member.
• handle_trigger_accept_(): retrieves and removes the constructor atomically via map::extract(), eliminating any window where a stale or overwritten entry could be consumed.

// Before — shared member, last writer wins
pending_tree_constructor_ = getTreeConstructorFromGoal(goal_ptr);   // handle_trigger_goal_
startExecution(pending_tree_constructor_, ...);                      // handle_trigger_accept_

// After — per-goal, keyed by UUID
pending_tree_constructors_[uuid] = getTreeConstructorFromGoal(goal_ptr);  // handle_trigger_goal_
auto node = pending_tree_constructors_.extract(uuid);                     // handle_trigger_accept_
startExecution(std::move(node.mapped()), ...);

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