fix: Adapt RAG example to use createAgent

[KavithaSiva]

Context

This PR adapts the current RAG example using the latest RAG agent tutorial.

It also fixes smoke-test failures by adding the missing @langchain/classic package.

Successful test run: https://github.com/SAP/ai-sdk-js/actions/runs/19854088195

[ZhongpinWang]

Perhaps we can adapt this RAG example and avoid using the more or less legacy usages. https://www.npmjs.com/package/@langchain/classic

[KavithaSiva]

Perhaps we can adapt this RAG example and avoid using the more or less legacy usages. https://www.npmjs.com/package/@langchain/classic

I have adapted the RAG sample to use createAgent based on the new RAG agent tutorial, so, I did manage to remove few imports from the classic package.

But some imports like MemoryVectorStore and TextLoader still require the langchain classic package.