refactor: registry package split

[akoumpa]

What does this PR do ?

• Splits monolithic _transformers/registry.py into a package:
  registry/base.py, model_package_spec.py, model_registry.py, and __init__.py.
• Introduces ModelPackageSpec as the shared metadata format for model registration, architectures, model types, and config registration.
• Keeps model classes lazy-loaded while moving lookup/indexing logic into _BaseModelRegistry.
• Adds a separate RetrievalModelRegistry and RETRIEVAL_MODEL_PACKAGE_SPECS.
• Moves bidirectional retrieval models out of the main model registry and into the retrieval registry.
• Updates retrieval code to use RetrievalModelRegistry directly.
• Removes register_retrieval() and has_retrieval_model().
• Updates model init and coverage tests to use the new registry package APIs.
• Expands tests for lazy loading, dynamic registration, config registration, registry exports, retrieval separation, and deprecation warnings.
• Validation run passed: 108 passed, 5 deselected.

Zoo refactor pt.1

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[akoumpa]

/ok to test e003c16

[akoumpa]

/ok to test c2e70b7