- carry result.contentChains through the graph-service response so downstream
  connectors (Telegram) can split steps into separate messages and hide
  tool_use without collapsing the reply into one bubble.
- ModelInitializer: set streaming:true so LangGraph streamEvents emits
  on_chat_model_stream even under model.invoke() (campaigns DefaultBehaviorNode)
  — live SSE deltas + tool_use boundaries; matches the Bedrock path.