Stream agent execution, yielding tuples matching Pregel's format.
Delegates to RemoteGraph.astream (which handles messages-tuple
negotiation, SSE routing, and namespace parsing) and converts the raw
message dicts into LangChain message objects for the adapter.
Whether to stream subgraph events.
LangGraph config with configurable.thread_id, etc.
Runtime context (e.g. CLIContext) forwarded to the
server via the SDK's context= parameter.
Ignored (server manages durability).
