feat: add gemini 2.5 & 2.5 flash lite previews for 09-25 - #232

To make it even easier to access our latest models while also reducing the need to keep track of long model string names, we are also introducing a -latest [alias](https://ai.google.dev/gemini-api/docs/models#latest) for each model family. This alias always points to our most recent model versions, allowing you to experiment with new features without needing to update your code for each release. You can access the new previews using:

    gemini-flash-latest

    gemini-flash-lite-latest