Typing `@` immediately after `(`, `[`, or `{` did not open the Agent
Panel’s @-mention completion menu, so `(@file)`, `[@file]`, and
`{@file}` were unusable. This has been bothering me for quite some time
now. Overall, I believe this is a QoL improvement, albeit a small one.

Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [ ] Unsafe blocks (if any) have justifying comments
- [ ] 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
- [ ] Performance impact has been considered and is acceptable

Release Notes:

- Fixed the Agent Panel’s @-mention menu not appearing when `@`
immediately follows `(`, `[`, or `{`.

---------

Co-authored-by: Bennet Bo Fenner <bennetbo@gmx.de>