Add DSL entity validation and transformation

[marot]

Overview

Fixes #215

Instead of adding an additional List.wrap, I thought it might be a good idea to perform validation and transformation early on. This PR removes one List.wrap :P

I believe this might cause a slow down of consumers, as we potentially recurse the entity tree repeatedly and don't cache the transformation results.

As a demo, this implements #215. But more validations could be added to catch common misconfigurations. (E.g. to also allow sub-entities without keys, by transforming entities: [@child] to entities: [child_entity_name: [@child]].

Please let me know if this is feasible/wanted - otherwise I'll just add the List.wrap :)

LLM generated commit message

Implements early validation of DSL entity structures to catch configuration errors at compile time rather than runtime. Key improvements:

• Add validate_and_transform/3 function for recursive entity validation
• Ensure nested entities are properly formatted as lists
• Add validation hooks in extension building, section processing, and patches
• Provide detailed error messages with module and path context
• Add test coverage for validation scenarios

This prevents invalid DSL configurations from causing runtime errors and provides better developer experience with clear error messages.

Contributor checklist

Leave anything that you believe does not apply unchecked.

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• Bug fixes include regression tests
• Chores
• Documentation changes
• Features include unit/acceptance tests
• Refactoring
• Update dependencies

[zachdaniel]

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