[SPARK-54938][PYTHON][TEST][FOLLOW-UP] Fix inferred time unit for pandas >= 3

[zhengruifeng]

What changes were proposed in this pull request?

Fix inferred time unit for pandas >= 3

Why are the changes needed?

there is behavior change in pandas 3

Does this PR introduce any user-facing change?

No, test-only

How was this patch tested?

manually check

pandas=2.3.3

In [7]: pd.__version__
Out[7]: '2.3.3'

In [8]: pd.Series(pd.to_datetime(["2024-01-01", "2024-01-02"])).dtype
Out[8]: dtype('<M8[ns]')

In [9]: pa.array(pd.Series(pd.to_datetime(["2024-01-01", "2024-01-02"]))).type
Out[9]: TimestampType(timestamp[ns])

pandas=3.0.1

In [6]: pd.__version__
Out[6]: '3.0.1'

In [7]: pd.Series(pd.to_datetime(["2024-01-01", "2024-01-02"])).dtype
Out[7]: dtype('<M8[us]')

In [8]: pa.array(pd.Series(pd.to_datetime(["2024-01-01", "2024-01-02"]))).type
Out[8]: TimestampType(timestamp[us])

Was this patch authored or co-authored using generative AI tooling?

Co-authored-by: Claude code (Opus 4.6)

[zhengruifeng]

cc @Yicong-Huang