Python AIMessageChunk now recognized in RemoteGraph streams

Patch Changes

• 63847a6: fix(langchain): recognize Python AIMessageChunk plain message objects from RemoteGraph streams

  Python langchain-core serializes streaming message chunks with type: "AIMessageChunk", while TypeScript langchain-core uses type: "ai". The toUIMessageStream adapter previously only matched the TypeScript form, silently dropping text deltas and tool-call events when streaming from Python LangGraph servers via RemoteGraph.