fix: make TableWidget import-safe when traitlets is missing

- Added a mock fallback for 'traitlets' in bigframes/display/anywidget.py to avoid NameError during class definition when the package is not installed.
- Moved 'pytest.importorskip' calls in tests/unit/display/test_anywidget.py to the top to ensure tests are skipped before attempting any imports from the module under test.
- Added noqa: E402 to suppress flake8 warnings for imports after importorskip.

Author: shuoweil

bigframes/display/anywidget.py

@@ -43,6 +43,10 @@
     _ANYWIDGET_INSTALLED = True
 except Exception:
     _ANYWIDGET_INSTALLED = False
+    # Define dummy traitlets to avoid NameError at class definition time
+    import unittest.mock
+
+    traitlets = unittest.mock.MagicMock()
 
 _WIDGET_BASE: type[Any]
 if _ANYWIDGET_INSTALLED:

tests/unit/display/test_anywidget.py

@@ -18,13 +18,13 @@
 import pandas as pd
 import pytest
 
-import bigframes.dataframe
-import bigframes.display.anywidget as anywidget
-
 # Skip if anywidget/traitlets not installed, though they should be in the dev env
 pytest.importorskip("anywidget")
 pytest.importorskip("traitlets")
 
+import bigframes.dataframe  # noqa: E402
+import bigframes.display.anywidget as anywidget  # noqa: E402
+
 
 class TestTableWidget:
     def test_navigation_to_invalid_page_resets_to_valid_page_without_deadlock(self):