AG2 sample uses OpenAI by default, inconsistent with other samples

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.