chore(deps-dev): bump @stellar/stellar-sdk in /agent

Bumps [@stellar/stellar-sdk](https://github.com/stellar/js-stellar-sdk) from 15.1.0 to 16.1.0.
- [Release notes](https://github.com/stellar/js-stellar-sdk/releases)
- [Changelog](https://github.com/stellar/js-stellar-sdk/blob/main/CHANGELOG.md)
- [Commits](stellar/js-stellar-sdk@v15.1.0...v16.1.0)

---
updated-dependencies:
- dependency-name: "@stellar/stellar-sdk"
  dependency-version: 16.1.0
  dependency-type: direct:development
  update-type: version-update:semver-major
...

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Author: dependabot[bot]