Merge develop: Hugging Face CI model downloads

[rmorse]

Summary

• Promotes the HF Hub based CI model download fix from develop to main.
• Pre-downloads the two test models with huggingface_hub using the repository HF_TOKEN secret.
• Keeps the vendored whisper.cpp download scripts untouched; cargo xtask test-setup now skips when models are already present.
• Adds model-file caching to reduce repeated Hugging Face resolver traffic.

Verification

• develop CI passed: https://github.com/operator-kit/whisper-cpp-plus-rs/actions/runs/27470310687
• PR CI passed before squash merge: https://github.com/operator-kit/whisper-cpp-plus-rs/actions/runs/27469802917

Merge strategy: normal merge commit into main, preserving the develop integration point.