fix(cookbook): scan macOS HF Library/Caches so Serve lists downloads

[michaelxer]

Summary

On macOS, Hugging Face downloads often land under ~/Library/Caches/huggingface/hub (and users may set HF_HOME to the hub directory itself). Serve's /api/model/cached scanner only looked under ~/.cache/huggingface/hub (plus a few env overrides), so freshly downloaded models never appeared in the Launch list even though the files were on disk.

This PR adds the macOS Library/Caches hub path, honors HF_HUB_CACHE, treats HF_HOME pointing at a hub directory as scannable, and also runs scan_hf for custom modelDirs that are themselves HF hub caches (models--*).

Target branch

• This PR targets dev, not main.

Linked Issue

Fixes #5978
Related: #5507 (custom modelDirs with HF hub layout)

Type of Change

• Bug fix (non-breaking — fixes a confirmed issue)
• New feature (non-breaking — adds new behaviour)
• Breaking change (changes or removes existing behaviour)
• Refactor / cleanup (behaviour unchanged)
• Documentation only
• CI / tooling / configuration

What changed

In routes/cookbook_helpers.py _cached_model_scan_script:

1. Add ~/Library/Caches/huggingface/hub to default HF cache candidates.
2. Also read HF_HUB_CACHE.
3. If HF_HOME is set, scan both $HF_HOME/hub and HF_HOME itself (when the env already points at a hub tree).
4. For each custom model_dir, call scan_hf(normalize_model_dir(...)) before scan_dir(...) so hub-layout directories are listed.

How to Test

python -m pytest \
  tests/test_cookbook_helpers.py::test_cached_model_scan_includes_macos_library_caches \
  tests/test_cookbook_helpers.py::test_cached_model_scan_hf_home_pointing_at_hub_dir \
  tests/test_cookbook_helpers.py::test_cached_model_scan_model_dir_with_hf_hub_layout \
  tests/test_cookbook_helpers.py::test_cached_model_scan_uses_huggingface_cache_env \
  tests/test_cookbook_helpers.py::test_cached_model_scan_runs_additional_hf_cache \
  tests/test_cookbook_helpers.py::test_cached_model_scan_reports_plain_dir_gguf \
  -q
# 6 passed

Manual (macOS):

1. Download a GGUF so it lives under ~/Library/Caches/huggingface/hub/models--….
2. Open Cookbook → Launch / cached models.
3. Confirm the model appears without needing ln -s into ~/.cache.

Checklist

• I searched open issues and open PRs — this is not a duplicate of an open PR for Cookbook won't find cached models when downloaded on MacOS #5978.
• This PR targets dev.
• Changes are limited to the cache scanner + focused regression tests.
• Full macOS app UI e2e not run on this machine (Linux VPS). Scanner behaviour is covered by unit tests that place a realistic HF hub tree under a fake HOME/Library/Caches/… path.

Visual / UI changes

None — backend scanner only.