refactor: remove legacy assistant compatibility

backend/services/config_resolver.py

@@ -21,7 +21,7 @@ from models import (
     RuntimeTool,
     RuntimeToolBinding,
 )
-from services.node_specs import graph_references, normalize_graph
+from services.node_specs import copy_graph, graph_references
 from sqlalchemy import select
 from sqlalchemy.ext.asyncio import AsyncSession
 
@@ -228,7 +228,7 @@ async def resolve_runtime_config(
         else None
     )
 
-    graph = normalize_graph(assistant.graph or {}) if assistant.type == "workflow" else {}
+    graph = copy_graph(assistant.graph or {}) if assistant.type == "workflow" else {}
     refs = graph_references(graph) if graph else {
         "model_resources": set(),
         "knowledge_bases": set(),