Update Cerebras model documentation

[ryanl-cerebras]

Summary

• Replaces retired Cerebras aliases with the three currently available public model IDs and updates the gateway example.
• The current public catalog is gpt-oss-120b, zai-glm-4.7, and gemma-4-31b.

Why

The previous model ID was retired or the fixed catalog was missing a currently available Cerebras model. This could produce failed requests, stale autocomplete, or misleading examples.

Validation

• MDX diff check and comparison with the live Cerebras model endpoint.
• Source of truth: https://api.cerebras.ai/public/v1/models

[cloudflare-docs-bot]

Review

✅ No issues found in commit 2e5eba5.

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