assistant: Limit model access for Zed AI users to Claude-3.5-sonnet (#15904)

This prevents users from accessing other models, such as OpenAI's GPT-4
or Google's Gemini-Pro.
Staff members can still access all models.

Co-authored-by: Thorsten <thorsten@zed.dev>

Release Notes:

- N/A

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Co-authored-by: Thorsten <thorsten@zed.dev>