[FEATURE]: Migrate from `google-generativeai` to `python-genai` and adopt `2.5-flash` models

[FallenDeity]

Problem Statement

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.

Proposed Solution

We should update our codebase (particularly llm.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.

Alternatives Considered

No response

Final Checklist

• This feature would be useful to other users
• I'm willing to help implement this feature