Closes https://github.com/zed-industries/zed/issues/57331

Model updates:
- **Free**: removed MiniMax M2.5 Free as per
https://github.com/anomalyco/models.dev/commit/10ddc78ce084dedb0fd62e96001b38303d382413
- **Zen**: added Gemini 3.5 Flash as per
https://github.com/anomalyco/models.dev/blob/f5f7d1a1677f549217b91575b7f28f969a15c3e2/providers/opencode/models/gemini-3.5-flash.toml
- **Zen**: added Grok Build 0.1 as per
https://github.com/anomalyco/models.dev/blob/f5f7d1a1677f549217b91575b7f28f969a15c3e2/providers/opencode/models/grok-build-0.1.toml

The newly-added Zen models **were not tested** as I don't have a Zen
subscription and I stubbornly refuse to get one.

Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [ ] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Release Notes:

- OpenCode: updated models (added Gemini 3.5 Flash and Grok Build 0.1,
removed MiniMax M2.5 Free)