Add LangGraph streaming adapter

[alkitrend]

What

Add a langGraphAdapter and langGraphMessageFormat in @openuidev/react-headless to enable LangGraph backend integration with OpenUI's streaming renderer and chat interfaces.

Changes

• Add langGraphAdapter in packages/react-headless/src/stream/adapters/langgraph.ts that parses LangGraph's named SSE events (messages, metadata, updates, error, end) and maps them to AG-UI events
• Add langGraphMessageFormat in packages/react-headless/src/stream/formats/langgraph-message-format.ts that converts between AG-UI messages and LangGraph's LangChain-style message format (human/ai/tool/system type discriminators, object-based tool call args, tool_call_id mapping)
• Support streaming text, streaming tool_call_chunks, complete tool_calls, and human-in-the-loop interrupts via onInterrupt callback
• Export langGraphAdapter, langGraphMessageFormat, LangGraphAdapterOptions, and LangGraphMessageFormat from package index
• Add 15 unit tests for the adapter
• Fix Prettier formatting issues from previous push

Test Plan

• Verified locally
• All 15 new adapter tests pass (text streaming, tool calls, errors, interrupts, chunked delivery)
• All 41 existing tests pass - no regressions
• TypeScript compiles with zero errors
• Prettier and ESLint pass on all changed files

Checklist

• I linked a related issue, if applicable
• I updated docs/README when needed
• I considered backwards compatibility

Closes #292

[abhithesys]

This is working as expected. We also need to add message converter to complete the langgraph integration.

[alkitrend]

This is working as expected. We also need to add message converter to complete the langgraph integration.

Gotcha, let me add it.

[abhithesys]

Looks good. We can later improve on how we handle other events eg. snapshot can update entire chat history.
lets merge this.