test: cover held-out COLMAP requirement

tests/test_import_colmap.py

@@ -1,4 +1,5 @@
 import json
+from unittest.mock import patch
 
 from import_colmap import import_colmap
 from validate_capture_manifest import validate_capture_manifest
@@ -34,3 +35,14 @@ def test_import_colmap_rejects_unknown_camera_model(tmp_path):
         assert "unsupported COLMAP camera model" in str(exc)
     else:
         raise AssertionError("unknown camera model should fail")
+
+
+def test_import_colmap_cli_requires_held_out_split():
+    from import_colmap import main
+    with patch("import_colmap.Path.is_file", return_value=False):
+        try:
+            main(["cameras.txt", "images.txt", ".", "capture.json"])
+        except SystemExit as exc:
+            assert exc.code == 2
+        else:
+            raise AssertionError("CLI should require held-out views")