Cover the unmatched model case and drop a redundant reset.

Reassigning the dropdown choices already resets the value, so the explicit
fallback never ran.

Author: aymuos15

cellfinder/napari/train/train.py

@@ -179,10 +179,7 @@ def widget(
 
     @widget.dimensions.changed.connect
     def update_pretrained_model_choices(dimensions: int):
-        choices = models_for_dimensions(dimensions)
-        widget.pretrained_model.choices = choices
-        if widget.pretrained_model.value not in choices:
-            widget.pretrained_model.value = choices[0]
+        widget.pretrained_model.choices = models_for_dimensions(dimensions)
 
     @widget.reset_button.changed.connect
     def restore_defaults():

tests/core/test_unit/test_download.py

@@ -53,6 +53,11 @@ def test_default_model_matches_mode(dimensions, has_background, expected):
     assert download.default_model(dimensions, has_background) == expected
 
 
+def test_default_model_without_a_registered_match():
+    with pytest.raises(ValueError, match="No pretrained model"):
+        download.default_model(4, True)
+
+
 def test_validate_model_dimensions_rejects_mismatch():
     with pytest.raises(ValueError, match="is 2D, but dimensions=3"):
         download.validate_model_dimensions("resnet50_2d", 3)