fix: Handle None workflow_id in success path

Also use 'N/A' placeholder in ExecutionResult when workflow_id is None
(when using graph_data directly instead of workflow_definitions).

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

backend/workflow_executor.py

@@ -186,7 +186,7 @@ async def execute_workflow(
 
         return ExecutionResult(
             execution_id=execution_id,
-            workflow_id=workflow_id,
+            workflow_id=workflow_id or "N/A",  # Placeholder when graph_data is used directly
             status="completed",
             node_states=node_states,
             aggregated_result=aggregated,