refactor: organize model registry and infrastructure packages - #4

Summary

Reorganizes the package around the model-registry store boundary. The store, repositories, persistence types, and domain errors now live under model_registry/. MongoDB settings and reusable array-update helpers live under infrastructure/.

mlflow_mongodb/
├── infrastructure/
│   ├── __init__.py
│   ├── _array_updates.py
│   └── settings.py
├── model_registry/
│   ├── __init__.py
│   ├── errors.py
│   ├── store.py
│   ├── types.py
│   └── repositories/
│       ├── __init__.py
│       ├── model_versions.py
│       └── registered_models.py
└── __init__.py

The idea is to organize the package by MLflow store boundary: tracking and model registry are separate interfaces, so each should own its store code and repositories. Only infrastructure they genuinely share belongs in infrastructure/.