* chore: migrate to vitest
* test: speed up some suites by sharing sessions and mocking expensive non-tested actions
* test: refactor some tests to verify behavior instead of implementation details
* chore: fix linting issues
<!-- This is an auto-generated comment: release notes by coderabbit.ai -->
## Summary by CodeRabbit
* **Tests**
* Improved test infrastructure and organization across backup, repository, volume, and utility modules with streamlined mock setup and cleaner test patterns.
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