Result from backend write operations.
Checkpoint backends populate filesUpdate with {file_path: file_data} for LangGraph state. External backends set filesUpdate to null (already persisted to disk/S3/database/etc).
Error message on failure, undefined on success
Metadata for the write operation, attached to the ToolMessage
File path of written file, undefined on failure
State update dict for checkpoint backends, null for external storage. Checkpoint backends populate this with {file_path: file_data} for LangGraph state. External backends set null (already persisted to disk/S3/database/etc).
