This PR changes the default fast model for the Zed provider from Claude
3.5 Haiku to Claude 3.5 Sonnet.

We don't offer Claude 3.5 Haiku to users.

Closes https://github.com/zed-industries/zed/issues/29505.

Release Notes:

- agent: Changed the default fast model for the Zed provider to Claude
3.5 Sonnet.