tests(bigframes): re-enable some tests blocking the doctest

[chelsea-lin]

Fixes internal tests 506201379🦕

[gemini-code-assist]

Code Review

This pull request updates doctests and unit tests to align with changes in struct field ordering and simplifies JSON comparison logic using pd.testing.assert_series_equal. However, the removal of check_dtype=False in test_series.py may lead to test failures or flakiness if the dtypes from the BigQuery backend do not perfectly match the pandas defaults.

[chalmerlowe]

@chelsea-lin

These appear in both Kokoro System and Kokoro Prerelease Tests:

FAILED tests/system/small/test_series.py::test_divmods_series[rdivmod-int64_col-int64_col]
FAILED tests/system/small/test_series.py::test_divmods_scalars[divmod--1000-int64_col]
FAILED tests/system/small/test_series.py::test_divmods_series[divmod-int64_col-int64_col]
FAILED tests/system/small/test_series.py::test_divmods_scalars[divmod-678-int64_col]
FAILED tests/system/small/test_series.py::test_divmods_scalars[rdivmod--1000-int64_col]
FAILED tests/system/small/test_series.py::test_divmods_scalars[rdivmod-678-int64_col]