fix: handle missing FAISS index gracefully instead of crashing

[Mimi-pan]

Fixes #312

On a fresh setup without running make build-data, the FAISS index does not exist yet. The chatbot would crash with a 500 error instead of returning a helpful response.

This PR adds a graceful fallback by checking whether the index file exists before attempting to load it, and returns a clear warning message when it is unavailable.

Files changed:

• vectorstore_utils.py — added existence check before loading FAISS index
• retriever_utils.py — return empty results when index or metadata is unavailable
• test_vectorstore_utils.py — updated tests to reflect new behavior

[berviantoleo]

Could you please run the linter in your local before pushing it here?

[j4b3-21]

hey this is a good pr .
this completely patch up the issue i raised.
thank a lot @Mimi-pan

[Mimi-pan]

Glad to hear that it worked! Thank you for the confirmation and the feedback.

[Mimi-pan]

@berviantoleo Hi! I've fixed the pylint issues as requested. All CI checks are passing now. Could you please take another look? 🙏