Bump thiserror from 2.0.4 to 2.0.6 in /backend in the cargo group

Bumps the cargo group in /backend with 1 update: [thiserror](https://github.com/dtolnay/thiserror).


Updates `thiserror` from 2.0.4 to 2.0.6
- [Release notes](https://github.com/dtolnay/thiserror/releases)
- [Commits](dtolnay/thiserror@2.0.4...2.0.6)

---
updated-dependencies:
- dependency-name: thiserror
  dependency-type: direct:production
  update-type: version-update:semver-patch
  dependency-group: cargo
...

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