fix: KB tree builder demotes decisions with < 2 branches to actions

Decision nodes with fewer than 2 buildable child targets now get
demoted to action nodes instead of creating invalid tree structures.
Also adds id fields to option objects (required by tree editor).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/api/endpoints/kb_accelerator.py

@@ -667,26 +667,60 @@ def _build_troubleshooting_tree(nodes: list[KBImportNode]) -> dict:
 
         # question/decision type — recursively build children
         options = content.get("options", [])
-        children = []
+
+        # Count how many options point to buildable (not-yet-placed) targets
+        buildable_targets = []
         for opt in options:
             next_id = opt.get("next_node_id")
+            if next_id and next_id in original_id_map and next_id not in placed:
+                buildable_targets.append(next_id)
+
+        # Decision nodes MUST have at least 2 branches to pass validation.
+        # If fewer than 2 buildable targets, demote to action node.
+        if len(buildable_targets) < 2:
+            demoted: dict = {
+                "id": node_id,
+                "type": "action",
+                "title": question_text,
+                "description": content.get("description", ""),
+            }
+            if buildable_targets:
+                demoted["next_node_id"] = buildable_targets[0]
+            elif options:
+                # All targets already placed; reference first option's target
+                first_next = options[0].get("next_node_id")
+                if first_next:
+                    demoted["next_node_id"] = first_next
+            return demoted
+
+        # Build children for decision node
+        children = []
+        built_options = []
+        for opt in options:
+            next_id = opt.get("next_node_id")
+            opt_id = opt.get("id", f"opt-{node_id}-{len(built_options)}")
             if next_id and next_id in original_id_map:
                 child_node = _build_node(original_id_map[next_id])
                 if child_node is not None:
                     children.append(child_node)
-                    # If the child is an action with a next_node_id, also
-                    # build that target as a sibling (the tree editor
-                    # expects reachable nodes nested under the decision)
                     _collect_action_chain(child_node, children)
+                built_options.append({
+                    "id": opt_id,
+                    "label": opt.get("label", ""),
+                    "next_node_id": next_id,
+                })
+            else:
+                built_options.append({
+                    "id": opt_id,
+                    "label": opt.get("label", ""),
+                    "next_node_id": next_id or "",
+                })
 
         return {
             "id": node_id,
             "type": "decision",
             "question": question_text,
-            "options": [
+            "options": built_options,
-                {"label": opt.get("label", ""), "next_node_id": opt.get("next_node_id", "")}
-                for opt in options
-            ],
             "children": children,
         }