Enables using Zeta edit predictions with a custom
`ZED_PREDICT_EDITS_URL` without requiring authentication to Zed servers.
This is useful for:

- Development and testing workflows
- Self-hosted Zeta instances
- Custom AI model endpoints

Prior context on this usage of `ZED_PREDICT_EDITS_URL`:
https://github.com/zed-industries/zed/pull/30418

Release Notes:
- Improved self-hosted zeta UX. Users no longer have to log into Zed to
use custom or self-hosted zeta backends.

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Co-authored-by: Agus Zubiaga <agus@zed.dev>