feat: add LlamaIndex retrievers

[raoabinav]

Summary

• add optional LlamaIndex retrievers backed by existing LEANN indexes
• support pure vector retrieval and hybrid dense-vector plus BM25 retrieval
• convert LEANN SearchResult values into LlamaIndex TextNode / NodeWithScore objects while preserving IDs, text, scores, and metadata
• document usage and add focused unit tests for scoring, weighting, query validation, async retrieval, and lazy integration imports

Review note

This overlaps with existing external PR #274 for issue #217. This PR is opened as a smaller adapter-focused implementation with tests/docs so maintainers can compare the approaches directly.

Tests

• PYTHONPATH=/Users/abinavrao/LEANN/.worktrees/p1-llamaindex-review/packages/leann-core/src:/Users/abinavrao/LEANN/.worktrees/p1-llamaindex-review/packages/leann-backend-hnsw /Users/abinavrao/LEANN/.worktrees/cid-migrate/.venv/bin/python -m pytest tests/test_llamaindex_integration.py -q
• /Users/abinavrao/LEANN/.worktrees/cid-migrate/.venv/bin/ruff check packages/leann-core/src/leann/integrations tests/test_llamaindex_integration.py
• /Users/abinavrao/LEANN/.worktrees/cid-migrate/.venv/bin/ruff format --check packages/leann-core/src/leann/integrations tests/test_llamaindex_integration.py
• git diff --check upstream/main...HEAD

Addresses #217.