fix(api): resolve graphon upgrade CI failures

Repair the plugin model runtime for Graphon 0.3.0 by implementing structured-output support, normalizing non-stream LLM calls, and tightening workflow-layer type safety.

Update the remaining workflow tests to use VariablePool.from_bootstrap(...) and the new node data constructor API so the CI unit and integration suites match the upgraded runtime behavior.

api/core/app/workflow/layers/llm_quota.py

@@ -156,7 +156,6 @@ class LLMQuotaLayer(GraphEngineLayer):
         except Exception:
             logger.exception("Failed to send quota abort command")
 
-    @staticmethod
     def _supports_quota(node: Node) -> bool:
         return node.node_type in _QUOTA_NODE_TYPES