Support validation of the `schema` in a CatalogItem

_↴ Ported from [flutter/genui#433](https://github.com/flutter/genui/issues/433) — originally opened by [polina-c](https://github.com/polina-c) on 2025-10-23._
_Original labels: P2, quality_

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Can we have the method Catalog.validate() that:

1. Invokes the existing validateCatalogExamples
2. Invokes validation of schema, i.e.:
- there are properties 'properties' and 'description'
- property 'required' contains only existing properties
- 'properties' is a map

Look for the Catalog class and its validate method or similar validation logic. The existing validateCatalogExamples function is a starting point. Examine the schema structure to check for 'properties' and 'description' keys, ensure 'required' contains only existing properties, and verify 'properties' is a map. Write tests to confirm the validation works as described.

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