Add LangChain vector sample

[shaowei6]

Adds a complete Dozer + LangChain sample for #1690.

Summary:

• Adds samples/llm-vector-langchain with customer profiles, transactions, and credit card product data.
• Includes a Dozer config that exposes the datasets as REST endpoints for the sample app.
• Builds a LangChain + Chroma recommendation flow, with OpenAI support when OPENAI_API_KEY is set and a deterministic local fallback for development.
• Adds offline tests for the recommendation/context-building helpers.

Testing:

• python -m unittest discover -s tests
• python -m py_compile app.py dozer_client.py recommender.py tests/test_recommender.py

/claim #1690
Closes #1690

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