Migrate a real legacy LangChain support RAG + router app across a full API break with hard-fail…

Migrate a real legacy LangChain support RAG + router app across a full API break with hard-fail…: a task in Long-Horizon Terminal-Bench (LHTB) (Harbor dataset). You are migrating a legacy LangChain support application across a full LangChain API break, from an extremely early runtime to the target…