fix: replace deprecated gemini-2.5-flash-lite-preview-06-17 with gemini-2.5-flash-lite
Updated all references to the deprecated Gemini model in:
- graphiti_core/llm_client/gemini_client.py
- graphiti_core/cross_encoder/gemini_reranker_client.py
- tests/llm_client/test_gemini_client.py
- README.md
This resolves 404 errors when using Gemini clients.
Fixes#1075🤖 Generated with [Claude Code](https://claude.ai/code)
Co-authored-by: claude[bot] <41898282+claude[bot]@users.noreply.github.com>
Co-authored-by: Daniel Chalef <danielchalef@users.noreply.github.com>
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