fix(cookbook): resolve MLX serve snapshots from the macOS HF cache (#5978)

[Spoofiecus]

Summary

The cookbook tmux runner resolves mlx-community/... models to a local HuggingFace snapshot before serving. Its candidate list only searched ~/.cache/huggingface/hub (Linux), so on macOS — where huggingface_hub defaults to ~/Library/Caches/huggingface/hub — a downloaded model was not found and the serve fell back to an unusable path. This is the same defect class as #5978 on the serve path; the scan path was fixed separately.

Target branch

• This PR targets **dev**, not main. All PRs land in dev.

Type of Change

• Bug fix (non-breaking — fixes a confirmed issue)

Linked Issue

Part of #5978

Checklist

• I searched open issues and open PRs — this is not a duplicate.
• This PR targets dev
• My changes are limited to the scope described above — no unrelated refactors or whitespace changes mixed in.
• I actually ran the app (uvicorn app:app, native venv) and verified the change works; details in How to Test.

How to Test

Regression proof (test fails before, passes after): git checkout -- routes/cookbook_routes.py then ./venv/bin/python -m pytest tests/test_cookbook_mlx_cache_roots.py -> the macOS and platform-difference tests FAIL; restore the fix -> 5 passed.
./venv/bin/python -m pytest tests/test_cookbook_mlx_cache_roots.py tests/test_cookbook_helpers.py tests/test_cookbook_stale_shim_recovery.py tests/test_cookbook_serve_cmd_js.py -> 102 passed, 1 skipped.
node --check n/a (no JS in this diff). ./venv/bin/python -m compileall -q app.py core routes src services scripts tests -> clean.
Live: :7001 and :7000 healthy; the runner script is generated per launch, so no served asset/cache-buster applies.
Pre-existing 4-test baseline failures are the workspace baseline (rule 30), unrelated to this diff.

Scope is the MLX serve-path cache candidate list plus its test. No backend API, arg-parsing, or UI change. The maintainer-specific absolute path at cookbook_routes.py:668 (a MiniMax-M3 remap) is a separate hygiene issue and is deliberately left alone here.