open_ai: Enable parallel tool calling for models that support it (#52203)

## Context

We seemed to have disabled it in #28056, because our agent did not
support parallel tool calls at the time.

## Self-Review Checklist

<!-- Check before requesting review: -->
- [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)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Release Notes:

- N/A or Added/Fixed/Improved ...
256135e18aBennet Bo Fenner committed on 3/23/2026, 1:48:06 PM· committed by GitHubparent45ca651
1 file changedLine totals unavailable