Resolves#45220 (one of several PRs, we are very close)
## Testing
- [X] QA'd all new/changed functionality manually
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## Summary by CodeRabbit
* **Tests**
* Improved test infrastructure for the CLI: consolidated and renamed
test helpers, added a dedicated in-process CLI test helper, and updated
many test cases to use the new helpers.
* Tightened several test assertions and standardized output/error
validation across unit and integration tests to improve reliability.
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[](https://app.coderabbit.ai/change-stack/fleetdm/fleet/pull/45585)
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