Add typed MeshModel summary API integration and tests

## 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)
5. Add unit tests for summary filters:
- total counts
- grouped counts by included dimensions
- validation failures for invalid dimensions