- Update _make_valid_tree() in test_ai_tree_validator to use next_node_id
on action nodes (solution is a sibling, not a child)
- Fix test_dead_end_action_node → test_dead_end_decision_node (action nodes
don't have child-based dead ends; dead ends are decision nodes with no children)
- Add test_action_missing_next_node_id for the new validation rule
- Update BRANCH_DETAIL_JSON in test_ai_endpoints to use next_node_id pattern
- Update test_draft_trees.py to use "title" field for action/solution nodes
(tree_validation.py was updated this branch to require "title" not "action"/"solution")
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>