docs: update web search model

[floze-the-genius]

Summary

• replace the discontinued openai/gpt-5.3-chat model in the web-search docs
• keep the inline example and runnable playground on openai/gpt-5.6-luna
• leave the separate OpenRouter provider-catalog smoke test unchanged

Validation

• npm ci
• npm run build in src/docs
• verified the generated docs and playground contain the new model ID

Closes #3558

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[reynaldichernando]

hi @floze-the-genius
this looks good, could you sign the CLA? it’s standard for contribution