Closes AC-1484. Verify that the key used to sign a synthetic transaction actually belongs to an operator.
## Review Checklist

**If any item is not complete, the merge request is not ready to be reviewed and must be marked `Draft:`.**

- [x] The merge request title is in the format `<change type>(<change scope>): <short description> [<task id>]`
  - For example, `feat(cli): add QR code generation [AC-123]`
  - For details, see [CONTRIBUTING.md](/CONTRIBUTING.md)
- [x] The description includes `Closes <jira task ID>` (or rarely `Updates <jira task ID>`)
- [x] The change is fully validated by tests that are run during CI
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  - In some cases, a Go test may be acceptable
  - Validation is not applicable to things like documentation updates
  - Purely UI/UX changes can be manually validated, such as changes to human-readable output
  - For all other changes, automated validation tests are an absolute requirement unless a maintainer specifically explains why they are not in a comment on this merge request
- [x] The change is marked with one of the validation labels
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  - ~"validation::manual" for changes validated by hand
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## Merge Checklist

- [ ] CI is passing
- [ ] Merge conflicts are resolved
- [ ] All discussions are resolved

Related to AC-1484