fix: update tests to match action node schema (next_node_id, not children)

- 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>

backend/tests/test_ai_tree_validator.py

@@ -5,7 +5,11 @@ from app.core.ai_tree_validator import validate_generated_tree, count_tree_stats
 
 
 def _make_valid_tree():
-    """Helper: minimal valid tree for testing."""
+    """Helper: minimal valid tree for testing.
+
+    Action nodes use next_node_id to point to a sibling (not children).
+    The solution following an action is a sibling under the parent decision.
+    """
     return {
         "id": "root",
         "type": "decision",
@@ -44,14 +48,13 @@ def _make_valid_tree():
                 "title": "Restart the Service",
                 "description": "Restart the service and verify.",
                 "commands": ["Restart-Service -Name 'TestService'"],
-                "children": [
-                    {
-                        "id": "service-resolved",
-                        "type": "solution",
-                        "title": "Service Restored",
-                        "description": "Service is running after restart.",
-                    },
-                ],
+                "next_node_id": "service-resolved",
+            },
+            {
+                "id": "service-resolved",
+                "type": "solution",
+                "title": "Service Restored",
+                "description": "Service is running after restart.",
             },
         ],
     }
@@ -107,6 +110,12 @@ class TestNodeValidation:
         errors = validate_generated_tree(tree)
         assert any("at least 2 options" in e for e in errors)
 
+    def test_action_missing_next_node_id(self):
+        tree = _make_valid_tree()
+        del tree["children"][1]["next_node_id"]
+        errors = validate_generated_tree(tree)
+        assert any("missing 'next_node_id'" in e for e in errors)
+
 
 class TestReferenceIntegrity:
     def test_option_references_nonexistent_child(self):
@@ -156,18 +165,18 @@ class TestGlobalChecks:
             {"id": "o1", "label": "A", "next_node_id": "only-solution"},
             {"id": "o2", "label": "B", "next_node_id": "only-solution"},
         ]
-        # Now restart-service branch has 1 solution, check-logs has 1 = total 2
-        # Remove one more to get to 1
-        tree["children"][1]["children"] = []
+        # Remove the solution that restart-service points to
+        tree["children"].pop(2)  # remove service-resolved
         errors = validate_generated_tree(tree)
         assert any("solution" in e.lower() for e in errors)
 
 
 class TestDeadEndDetection:
-    def test_dead_end_action_node(self):
+    def test_dead_end_decision_node(self):
+        """A decision node with no children is a dead end."""
         tree = _make_valid_tree()
-        # Remove restart-service's children — becomes dead end
-        tree["children"][1]["children"] = []
+        # Remove children from check-logs decision node — becomes dead end
+        tree["children"][0]["children"] = []
         errors = validate_generated_tree(tree)
         assert any("dead end" in e for e in errors)