Resolves#36087 (one of several PRs).
## Testing
- [x] QA'd all new/changed functionality manually.
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## Summary by CodeRabbit
* **New Features**
* Dry-run support when creating secret variables.
* **Improvements**
* Standardized API models for secret-variables and targets for more
consistent behavior.
* List secret variables now includes pagination metadata.
* More consistent error reporting across secret-variables and targets
APIs.
* Target search/count behavior refined: pre-selected built-in labels are
omitted as expected.
* **Tests**
* Integration tests updated to validate the new request/response
behavior and target-selection logic.
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[](https://app.coderabbit.ai/change-stack/fleetdm/fleet/pull/46196?utm_source=github_walkthrough&utm_medium=github&utm_campaign=change_stack)
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