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

---------

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

crates/collab/src/rpc.rs

@@ -5164,6 +5164,7 @@ async fn get_llm_api_token(
 
     let token = LlmTokenClaims::create(
         session.user_id(),
+        session.is_staff(),
         session.current_plan().await?,
         &session.app_state.config,
     )?;