Add Gemma mode

[emagda97]

The earliest version of Genkit that has the Gemma models in it is 1.14.1.

Seems that their implementation doesn't support JSON mode yet.

So there's slight parsing that needed to be done.

Seems like other folks were having the same problem:

https://discuss.ai.google.dev/t/gemma-3-missing-features-despite-announcement/71692/14

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[jfschmakeit]

@nohe427 - I think we resolved it in the chat, but yes - it looks like structured output isn't currently supported for Gemma in Genkit (at the moment?).

I agree that this is not ideal and potentially a little fragile, but including the response schema in the prompt seems to be working quite well for the current version of Gemma (based on my limited testing).

Happy for this to get merged, but let's put it in its own branch (gemma ?) - could you create it and switch the PR over to it @nohe427 ? Thank you for your help! 😄