Add support for cache_model_artifact

[shchur]

Issue #, if available:

Summary

Adds FoundationModel.cache_model_artifact() to bundle HuggingFace weights with the serve script into a SageMaker model.tar.gz and upload to S3, then reuse that artifact on deploy(). Required for network-isolated endpoints (e.g., SageMaker Serverless Inference) that can't reach HuggingFace at runtime.

Changes

• cache_model_artifact(cache_path, *, overwrite=False) — downloads weights via huggingface_hub.snapshot_download, packages weights/ + code/<serve_script> into model.tar.gz, uploads to {cache_path}/{model_id}/model.tar.gz. Returns a new FoundationModel with model_artifact_uri set. Skips upload if the key already exists; raises RuntimeError if the cached artifact's autogluon-cloud-version metadata doesn't match the current version (unless overwrite=True).
• SagemakerBackend.deploy(..., repack: bool = True) — when False, uses AutoGluonNonRepackInferenceModel directly so a pre-bundled tarball isn't downloaded, repacked, and re-uploaded by the SDK.
• FM deploy wiring — _deploy_backend passes model_artifact_uri as predictor_path, sets repack=False, and overrides hyperparameters["model_path"] to /opt/ml/model/weights (raises if the user also set model_path).
• Registry refactor — FoundationModelConfig is now a frozen dataclass with sensible instance-type defaults; renamed model_name → ag_model_key and added model_source_uri. Registry shrunk to chronos-bolt-{tiny,small,base} and chronos-2{,-small} (Mitra entries removed for now).
• Naming — serve_config / AG_SERVE_CONFIG → fm_serve_config / AG_FM_SERVE_CONFIG.
• Serialization — to_dict / to_json / from_dict / from_json on FoundationModel, excluding runtime context (role, cloud_output_path) so configs can be shared across users.
• Dependency — huggingface_hub>=0.20,<2 added to install_requires.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[github-actions]

Job PR-241-16627d5 is done.
Docs are uploaded to https://d12sc05jpx1wj5.cloudfront.net/PR-241/16627d5/index.html

[github-actions]

Job PR-241-5b5fc59 is done.
Docs are uploaded to https://d12sc05jpx1wj5.cloudfront.net/PR-241/5b5fc59/index.html