chore: use vercel AI SDK instead of langchain

LangChain's ToolCalling agent was not providing a structured tool call
response and different model providers were providing entirely different
tool calls for the same tool definition which was too turbulent for us
to have any accuracy baseline at all.

Vercel's AI SDK pushes us forward on that problem and the tool call
responses so far have always been well structured.

This commit replaces LangChain based implementation with Vercel's AI SDK
based implementation.

Author: himanshusinghs