refactor: extract PROCEDURAL_TREE_TYPES constant and add negative tree_type test

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

backend/app/core/tree_validation.py

@@ -1,6 +1,8 @@
 """Tree validation helper module for draft/published workflow."""
 from typing import Any
 
+PROCEDURAL_TREE_TYPES = {"procedural", "maintenance"}
+
 
 class TreeValidationError(Exception):
     """Custom exception for tree validation errors."""
@@ -224,14 +226,14 @@ def can_publish_tree(
         errors.append({"field": "name", "message": "Tree must have a name to be published"})
 
     # Validate structure based on tree type
-    if tree_type in ("procedural", "maintenance"):
+    if tree_type in PROCEDURAL_TREE_TYPES:
         structure_valid, structure_errors = validate_procedural_structure(tree_structure)
     else:
         structure_valid, structure_errors = validate_tree_structure(tree_structure)
     errors.extend(structure_errors)
 
     # Validate intake form if present (procedural only)
-    if intake_form and tree_type in ("procedural", "maintenance"):
+    if intake_form and tree_type in PROCEDURAL_TREE_TYPES:
         form_valid, form_errors = _validate_intake_form(intake_form)
         errors.extend(form_errors)