test(quant): skip declined local checkpoints and label cache paths portably

[YevheniiKotyrlo]

Problem

test_scheme_for_agrees_with_the_stored_tensors runs over every checkpoint in the local Hugging Face cache and /mnt/nvme/models, so what is on the machine decides whether it passes. Two things fail it for reasons outside the code under test:

1. A checkpoint whose quantization method FreeToken supports but whose scheme it declines - here a compressed-tensors W4A16 snapshot (int4, group 128) - raises NotImplementedError: compressed-tensors weight scheme {...} is not supported from QuantConfig.from_hf. The test expects that only for methods in _UNSUPPORTED.
2. _label finds the repo with models--([^/]+)/snapshots, which never matches a Windows path, so every cache entry is labelled nvme:<snapshot hash>.

Solution

A NotImplementedError from from_hf skips the row and says why - a clean decline is the behaviour a declined scheme should have, and the skip keeps it visible under -rs. _label reads the repo from the path's parts, which is right on every platform.

Verification

Windows 11, with the W4A16 snapshot in the cache: before, 1 failed as [nvme:124c14e7e8c7d2f5402933b9af368e772a9fcf0c[index-only]]; after, the row is [syvai/qwen3.8-27b-3090-fast-variant[index-only]] and skips with FreeToken declines this checkpoint: compressed-tensors weight scheme {...} is not supported. pytest tests/models/test_quant_config.py: 11 passed, 42 skipped; on Linux, whose cache holds no declined checkpoint, 11 passed and 42 skipped with or without this change.