Fix openai stream js snippet

[Wauplin]

Originally from @cfahlgren1 in moon-landing (private link):

In inference snippets

• [Bug] We don't add stream flag on javascript / openai snippet
• [Bug] We pass provider param in openai api which causes error
• [Improvement] Maybe we should have example not have \n after each chunk

EXample: https://huggingface.co/deepseek-ai/DeepSeek-V3-0324?inference_api=true&inference_provider=fireworks-ai&language=js

This PR addresses all 3 points. Fix is in https://github.com/huggingface/huggingface.js/pull/1344/files#diff-593c31a95f2e065cf06d977764fdc415600f67c5cc4a8204d73337ff6efd99c3.

Originally from @gary149 on slack (private link):

micro nit: "max_tokens": 512 everywhere instead of 500 (500 looks quite weird imo)

This PR addresses that as well.

[hanouticelina]

thanks for fixing this!