feat: add LlamaIndex retrievers

[raoabinav]

Evaluator Summary

• Abi added optional LlamaIndex retriever adapters backed by LEANN indexes, supporting both pure vector retrieval and hybrid dense-vector plus BM25 retrieval.
• The design keeps LlamaIndex as an optional lazy import, preserving lightweight core LEANN installs while enabling users already in the LlamaIndex ecosystem to plug in LEANN retrieval directly.
• The implementation converts LEANN SearchResult values into LlamaIndex TextNode / NodeWithScore objects while preserving IDs, text, metadata, and scores.
• Quality bar: tests cover query validation, weighting, scoring, metadata preservation, async retrieval, and lazy integration imports.

[raoabinav]

Parking this PR for now to keep the review queue focused on #347. I’ll revisit/reopen this work later as a smaller clean PR once the content-hash PR lands.