chore: make playground example fallback to WorkersAI when OpenAI key is not set

[iTrooz]

This PR allows you to use the playground example without an OpenAI key, by adding a fallback to WorkersAI

I'm already using it locally to test other PRs I'm submitting to this repository since I don't want to be billed

I figured the playground example was the most appropriate for this additions, since it's the most general

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Latest commit: 62636d6

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commit: 62636d6

[threepointone]

very nice, thanks!