Fix broken LangChain imports in examples/langchain_rag

[AlexFoxalt]

Name and Version

development

What is the problem this feature will solve?

examples/langchain_rag/app.py uses outdated LangChain import paths that are incompatible with currently pinned dependencies in examples/langchain_rag/requirements.txt.

Repro steps:

1. cd examples/langchain_rag
2. python -m venv .venv && source .venv/bin/activate
3. pip install -r requirements.txt
4. python app.py

Current result: app fails at import time with errors like:

• ModuleNotFoundError: No module named 'langchain.chains'
• ImportError: cannot import name 'CacheBackedEmbeddings' from 'langchain.embeddings'
• ModuleNotFoundError: No module named 'langchain.globals'
• ModuleNotFoundError: No module named 'langchain.storage'

Expected result: example should start without import-time failures (runtime config errors like missing DIAL_URL are acceptable).

What is the feature you are proposing to solve the problem?

Update examples/langchain_rag/app.py imports to match modern LangChain package split used by pinned versions:

• RetrievalQA import: langchain.chains... -> langchain_classic.chains.retrieval_qa.base
• CacheBackedEmbeddings import: langchain.embeddings -> langchain_classic.embeddings
• set_debug import: langchain.globals -> langchain_core.globals
• LocalFileStore import: langchain.storage -> langchain_classic.storage

Acceptance criteria:

• python app.py no longer fails on import errors in examples/langchain_rag.

What alternatives have you considered?

No response

[francois-maarbess]

Hi! I'm a software engineering student looking to make my first open-source contribution. I can update these import paths. Could you assign this to me?

[AlexFoxalt]

Hello, @francois-maarbess
Feel free to open a new pull request, you don't need any assignment for that.