Add LangChain module to Agentic AI course

[mukesh-dev-git]

Problem

The Agentic AI course needs a LangChain module after the first AI/LLM foundations module.

Scope

• Add a new LangChain module after Module 1.
• Source beginner topic structure from current LangChain documentation.
• Include topics for setup, chat models/providers, prompt templates, LCEL chains/output parsers, and agents/tools.
• Use the existing Agentic AI lesson factory so lessons, IDE exercises, and quizzes are generated consistently.

Acceptance criteria

• LangChain appears as the second Agentic AI module.
• All LangChain topics have lesson content, infographic data, IDE exercises, and generated quizzes.
• Existing Agentic AI topic IDs and routes remain stable.
• TypeScript checks pass.