Migrate from `google-generativeai` to `python-genai` and adopt flash-2.5 models

[FallenDeity]

With the release of Gemini 2.0, the google-generativeai Python SDK has been deprecated. Google now recommends migrating to the new unified python-genai SDK. We should update our codebase (particularly tools/llm_api.py) to use this new SDK, and adopt the latest flash-2.5 series models for improved performance and results.

I can make a PR for this if it sounds good.

[grapeot]

Yes that makes total sense. Feel free to submit a PR. Thanks a lot!