fix: explicit cleanup of temporary directory in DummyDatastore

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Describe your changes

Adds a __del__ method to the DummyDatastore class to explicitly call cleanup() on its TemporaryDirectory object. This prevents ResourceWarning: Implicitly cleaning up <TemporaryDirectory ...> that currently litters the test output when the datastore objects are garbage collected.

No new dependencies are required.

Issue Link

closes #486

Type of change

• 🐛 Bug fix (non-breaking change that fixes an issue)
• ✨ New feature (non-breaking change that adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to not work as expected)
• 📖 Documentation (Addition or improvements to documentation)

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