Bump thiserror from 1.0.26 to 1.0.28 in /src/agent (#1203)

Bumps [thiserror](https://github.com/dtolnay/thiserror) from 1.0.26 to 1.0.28.
- [Release notes](https://github.com/dtolnay/thiserror/releases)
- [Commits](dtolnay/thiserror@1.0.26...1.0.28)

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

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>