fix(langchain-agents): accept sync tool handlers

[andrewklatzke]

Summary

• LangChain agent tools now run both synchronous and asynchronous handlers. Graph __handoff_* tools stay synchronous so routing records the selected edge instead of raising when the result is awaited.
• Invoke and stream share _build_agent_tools(), so both routes pick up the same behavior. The graph handoff functions themselves are unchanged.

Fixes AIC-3507.

Test plan

• A synchronous handler returning a string works
• An asynchronous handler is still awaited
• A synthetic __handoff_* handler records and returns its destination
• Exceptions from either handler type propagate
• Invoke and streaming routes both execute a sync handoff
• Full Python suite: 1402 passed, 11 skipped

Made with Cursor

[andrewklatzke]

Human message:

Recreated the error on a minimal setup (three nodes, one handoff to each). Made the update then re-ran the graph that was previously failing on the error successfully (it even returned the structured output correctly as configured):