feat(gemini): add support for flex tier

https://ai.google.dev/gemini-api/docs/flex-inference

Same as OpenAI's flex tier

Thanks in advance!

Look for the Gemini API client code in the codebase, likely under a `gemini` module. Examine how other tiers (like OpenAI's flex tier) are implemented to understand the pattern. The change involves adding a new configuration option for the flex tier and ensuring it's correctly passed in API requests. Check existing tests for the Gemini module to see how to validate the new feature.

Rédigé par le modèle d'indexation à partir du texte de l'issue.