Update ChatCerebras models and default

[ryanl-cerebras]

Summary

• Updates the ChatCerebras selector to the three current public models and changes the default from retired llama3.1-8b to gpt-oss-120b.
• 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

• JSON parsing, git diff check, and an exact catalog comparison with the live Cerebras model endpoint.
• Source of truth: https://api.cerebras.ai/public/v1/models

[gemini-code-assist]

Caution

The consumer version of Gemini Code Assist on GitHub has been sunset. All code review activity has officially ceased.