Move unit tests from tests/unit/ to internal/ following Go conventions:
- tests/unit/config/* -> internal/config/* (constants, mode_paths, paths, validation)
- tests/unit/container/* -> internal/container/* (podman, security tests)
- tests/unit/envpool/* -> internal/envpool/* (envpool tests)
- tests/unit/errors/* -> internal/errtypes/* (errors_test.go moved to errtypes package)
- tests/unit/experiment/* -> internal/experiment/* (manager tests)
- tests/unit/jupyter/* -> internal/jupyter/* (config, package_blacklist, service_manager, trash_restore)
Update import paths in test files to reflect new locations.
Note: errors_test.go moved from tests/unit/errors/ to internal/errtypes/ to match the package structure.
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