feat: align runs, store, threads and assistant APIs with LangGraph Platform contract

[TBice123123]

Summary

• Align runs, store, threads, and assistant API endpoints with the LangGraph Platform contract (response models,status codes, streaming behavior)
• Fix delete run endpoint to return 204 No Content instead of 200
• Fix test assertions for error streaming format, response_model_exclude_none behavior, and cross-user thread
  access