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 memvid/llm_client.py) to use this new SDK, and adopt the latest flash-2.5 series models for improved performance and results.

memvid/memvid/llm_client.py

Line 20 in a8c0516

import google.generativeai as genai

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

[abhayKashyap03]

hello, if no one's working on this, i can take it up.