Add typed MeshModel summary API integration and tests

[Amr-Shams]

Problem

MeshModel summary filters exist, but registry-level integration and test coverage are incomplete.
We need a clean way to query summaries (components and relationships) without forcing summary filters through GetEntities, while avoiding package import cycles.

Proposal

1. Add/standardize a summary filter contract in models/meshmodel/entity.
2. Expose typed summary methods on RegistryManager:
   • GetComponentSummary(*v1beta1.ComponentSummaryFilter)
   • GetRelationshipSummary(*v1alpha3.RelationshipSummaryFilter)
3. Add unit tests for summary filters:
   • total counts
   • grouped counts by included dimensions
   • validation failures for invalid dimensions

[welcome]

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