Move checksummed and not_checksummed to VerifiableModel

What does this MR do and why?

Move checksummed and not_checksummed to VerifiableModel

The concept of "checksum" is in Geo should only apply to primary models, not secondary registries, which are verified. The MR:

• refactors the checksummed and not_checksummed out of the generic VerificationState class in favour of the more specific VerifiableModel;
• replaces the scopes override in the models with generic scopes inside VerifiableModel;
• extend the verifiable model shared examples to apply to verifiable models which do not have a separate state table. This ensures tests apply to both cases.

References

• Internal slack discussion: https://gitlab.slack.com/archives/C7U95P909/p1760015976915399
• Issue: #576325 (closed)

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