feat: allow 'answer' type in tree drafts, block on publish

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

backend/app/core/tree_validation.py

@@ -53,6 +53,13 @@ def validate_tree_structure(tree_structure: dict[str, Any]) -> tuple[bool, list[
     if "children" in tree_structure:
         _validate_children(tree_structure["children"], "root.children", errors)
 
+    # Block publish if any answer placeholder nodes remain
+    if _has_answer_nodes(tree_structure):
+        errors.append({
+            "field": "tree_structure",
+            "message": "Answer placeholders must be resolved to a node type before publishing."
+        })
+
     return len(errors) == 0, errors
 
 
@@ -89,6 +96,10 @@ def _validate_node(node: dict[str, Any], path: str, errors: list[dict[str, str]]
                 "message": "Solution nodes must have a non-empty solution"
             })
 
+    elif node_type == "answer":
+        # Answer nodes are draft-only placeholders — no structural validation needed
+        pass
+
     else:
         errors.append({
             "field": f"{path}.type",
@@ -115,6 +126,16 @@ def _validate_children(children: list[dict[str, Any]], path: str, errors: list[d
             _validate_children(child["children"], f"{child_path}.children", errors)
 
 
+def _has_answer_nodes(node: dict[str, Any]) -> bool:
+    """Recursively check if any node in the tree has type 'answer'."""
+    if node.get("type") == "answer":
+        return True
+    for child in node.get("children", []):
+        if _has_answer_nodes(child):
+            return True
+    return False
+
+
 # --- Procedural Tree Validation ---
 
 VALID_STEP_TYPES = {"procedure_step", "procedure_end", "section_header"}