fix: image model ranking crashes when system is not a dict (#1900)

tests/test_image_models_nondict_system.py

@@ -0,0 +1,9 @@
+from services.hwfit.image_models import rank_image_models, IMAGE_MODEL_REGISTRY
+
+
+def test_rank_image_models_handles_non_dict_system():
+    # `system` is the detected-hardware dict; if detection failed and returned
+    # None (or a non-dict), system.get(...) raised AttributeError. Treat a
+    # non-dict system as "unknown hardware" (no GPU) rather than crashing.
+    assert len(rank_image_models(None)) == len(IMAGE_MODEL_REGISTRY)
+    assert len(rank_image_models(123)) == len(IMAGE_MODEL_REGISTRY)