Add support for base64 images in chat messages

[celestial-rose]

Handle base64 image data for Llama 4 in chat messages.
As per the documentation found here https://developers.cloudflare.com/workers-ai/models/llama-4-scout-17b-16e-instruct/

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[threepointone]

what do you want to do with this PR?

[celestial-rose]

what do you want to do with this PR?

Wow I had in my todo to go back to these PRs later on today, so thank you for that

There's been some undocumented changes to the Workers AI API for Llama4 lately that occurred after this PR. The change provided a little more compliance towards the OpenAI Reponses API but it's still not totally the case
I am running an app in prod with a patch of this PR but I'm wondering if there isn't a better generic way to do that than my proposition here.