chore: fix unit tests for _read_gbq_colab when Polars isn't installed (#1966)

Author: tswast

tests/unit/pandas/io/test_api.py

@@ -14,10 +14,15 @@
 
 from unittest import mock
 
+import pytest
+
 import bigframes.dataframe
 import bigframes.pandas.io.api as bf_io_api
 import bigframes.session
 
+# _read_gbq_colab requires the polars engine.
+pytest.importorskip("polars")
+
 
 @mock.patch(
     "bigframes.pandas.io.api._set_default_session_location_if_possible_deferred_query"