test(compat): fail when an mlx_vlm patch target disappears instead of skipping

Every mlx_vlm compat patch try/except-imports its target and returns quietly
if the module, class or method is not where it expects. That is right for
robustness and wrong for testability: when upstream renames something the
patch becomes a no-op, and the entire LFM2/Qwen VL compat test file goes GREEN
by skipping, because all twelve of its tests use importorskip.

Users would find out instead of us — LFM2-VL image prompts dying on a
signature mismatch, bundles with llama-style MLP names refusing to load.

This test fails closed. mlx_vlm absent entirely still skips, which is a
legitimate dev environment. mlx_vlm present but the patch did not take is now
a FAILURE, because that means upstream moved and the behaviour we depend on
is silently gone.

Verified it actually fails: pointed the check at a sentinel that does not
exist and it reported the patch as not taken, rather than passing.