Bump the testing-stack group in /backend with 2 updates

Bumps the testing-stack group in /backend with 2 updates: [@vitest/coverage-v8](https://github.com/vitest-dev/vitest/tree/HEAD/packages/coverage-v8) and [vitest](https://github.com/vitest-dev/vitest/tree/HEAD/packages/vitest).


Updates `@vitest/coverage-v8` from 3.2.4 to 4.0.6
- [Release notes](https://github.com/vitest-dev/vitest/releases)
- [Commits](https://github.com/vitest-dev/vitest/commits/v4.0.6/packages/coverage-v8)

Updates `vitest` from 3.2.4 to 4.0.6
- [Release notes](https://github.com/vitest-dev/vitest/releases)
- [Commits](https://github.com/vitest-dev/vitest/commits/v4.0.6/packages/vitest)

---
updated-dependencies:
- dependency-name: "@vitest/coverage-v8"
  dependency-version: 4.0.6
  dependency-type: direct:development
  update-type: version-update:semver-major
  dependency-group: testing-stack
- dependency-name: vitest
  dependency-version: 4.0.6
  dependency-type: direct:development
  update-type: version-update:semver-major
  dependency-group: testing-stack
...

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

Author: dependabot[bot]