download_huggingface_dataset hardcodes repo_type="model", so it cannot fetch datasets repos

[MaxGhenis]

policyengine_core.tools.hugging_face.download_huggingface_dataset (policyengine-core 3.32.5, policyengine_core/tools/hugging_face.py:92-94) always calls hf_hub_download(..., repo_type="model"). Run against the canonical US population release policyengine/populace-us (a Hugging Face datasets repo; certified tag populace-us-2024-spm-20260915), it raises RepositoryNotFoundError: 401, because there is no model repo of that name. Observed today while re-qualifying the PolicyEngine 6.0.0 bundle (registry re-qualification lane, 2026-09-15); the wrapper policyengine 6.0.0 avoids the helper by reading repo_type: "dataset" from its bundle manifest and downloading with the correct type, and recomputing the certified hash independently.

Suggested fix: accept a repo_type argument (default "model" for backward compatibility with policyengine-uk-data-private and the other model-typed data repos), or infer it by probing repo_info for both types; pass it through to hf_hub_download and to the private/gated check. This is separate from #538, which concerns token handling for public-but-gated repos.

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