fix(tests): assert composite color via Pillow for IM6 CI

ImageMagick 6 convert cannot run `identify` as a subcommand; use Pillow
mode so Gitea runners without magick pass the composite color check.

Co-authored-by: Cursor <cursoragent@cursor.com>

tests/test_workflow_modules.py

@@ -340,8 +340,10 @@ class TestCompositeColor:
 
         output = output_dir / "fg.png"
         assert output.is_file()
-        result = run_command([magick, "identify", "-format", "%[type]", str(output)])
-        assert result.stdout.strip() != "Grayscale"
+        from PIL import Image
+
+        with Image.open(output) as img:
+            assert img.mode != "L"
 
 
 has_workflow_tools = check_workflow_tools()