Add LlamaIndex integration example

[kmadan]

Goal

Create examples/llamaindex-app/ showing how to capture LlamaIndex query-engine runs into an AICertify contract and evaluate them.

Why this matters

LlamaIndex is the second most common LLM framework for retrieval-heavy applications. RAG systems have specific compliance concerns (data leakage, hallucination, source citation) that AICertify policies can check.

Files likely involved

• New: examples/llamaindex-app/ (six-file shape)
• Optionally: a CallbackHandler subclass under aicertify/integrations/llamaindex.py
• examples/README.md table entry

Acceptance criteria

• A runnable LlamaIndex query engine over a small fixture corpus
• Each query + response is captured into a contract with retrieved-sources metadata
• Policy evaluation includes content-safety + transparency (source citation)
• expected_report.md describes both the pass case and common RAG failure modes (hallucinated citations, leaked private docs)

Help

Comment here if you want to work on this and I will help you get started. First-time contributors are welcome — open a draft PR early and I'll review.