fix(datasets): removed is_public

[briangreunke]

Summary

This PR addresses the removal of the is_public field from the DatasetMetadata model.

Changes

API Models

• Removed is_public field: The is_public: bool field has been removed from the DatasetMetadata Pydantic model in dreadnode/api/models.py. This impacts how dataset visibility is represented and handled within the API.

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Generated Summary:

• Simplified error message in the _validate_key function for clarity by removing unnecessary line breaks.
• Removed the is_public field from DatasetMetadata, which may affect how dataset visibility is handled in the application.
• These changes improve code readability and may require updates in related components that depend on the is_public attribute.

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