AG2 sample uses OpenAI by default, inconsistent with other samples #454
Description
The AG2 agent sample (samples/python/agents/ag2/) is hardcoded to use OpenAI (gpt-4o-mini) while most other samples in this repo default to Google Gemini via GOOGLE_API_KEY.
This means users following the demo UI setup (which provides a GOOGLE_API_KEY) cannot use the AG2 sample without also configuring a separate OpenAI key.
Switching the default to Gemini (with OpenAI as fallback) would make the AG2 sample consistent with the rest of the repo and simplify the getting-started experience.