Bump flat and mocha in /sandbox

Bumps [flat](https://github.com/hughsk/flat) to 5.0.2 and updates ancestor dependency [mocha](https://github.com/mochajs/mocha). These dependencies need to be updated together.


Updates `flat` from 4.1.1 to 5.0.2
- [Release notes](https://github.com/hughsk/flat/releases)
- [Commits](hughsk/flat@4.1.1...5.0.2)

Updates `mocha` from 7.2.0 to 11.7.1
- [Release notes](https://github.com/mochajs/mocha/releases)
- [Changelog](https://github.com/mochajs/mocha/blob/main/CHANGELOG.md)
- [Commits](mochajs/mocha@v7.2.0...v11.7.1)

---
updated-dependencies:
- dependency-name: flat
  dependency-version: 5.0.2
  dependency-type: indirect
- dependency-name: mocha
  dependency-version: 11.7.1
  dependency-type: direct:development
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]