GH-50464: [C++][Python] Simplify arrow_to_pandas DateOffset handling for nanoseconds/milliseconds

[raulcd]

Rationale for this change

Pandas 1.4 fixed the following:

• BUG: DateOffset with nanoseconds does not apply to Timestamp pandas-dev/pandas#43892

Which made us special case microseconds extracting those from our MonthDayNano and manually building microseconds.

DateOffset can be created with the following:

DateOffset(years=1, months=1,
                   days=1, seconds=1, microseconds=1,
                   minutes=1, hours=1, weeks=1, nanoseconds=1)

We currently only roundtrip Month, Day, Microseconds and Nano due to our MonthDayNano data type. We special cased microseconds but we were not special casing years, seconds, minutos, hours or weeks.

As the initial issue that made us special case has been solved upstream we can stop roundtripping microseconds and just return nanoseconds simplifying our codebase.

What changes are included in this PR?

Remove the unnecessary code to retrieve microseconds from nanoseconds.
Update test as microseconds won't be returned, only nanoseconds.

Are these changes tested?

Yes via CI

Are there any user-facing changes?

Yes, this is a change of the UX.

• GitHub Issue: [Python] Simplify arrow_to_pandas DateOffset handling for nanoseconds/milliseconds #50464

[github-actions]

⚠️ GitHub issue #50464 has been automatically assigned in GitHub to PR creator.

[raulcd]

@jorisvandenbossche would there be any reason to keep roundtriping microseconds on DateOffset(months=1, days=1, microseconds=1, nanoseconds=1) when we don't do it for years, seconds, minutes, hours, weeks?
We can simplify a little here with this PR.

[Copilot]

Pull request overview

This PR simplifies the Pandas DateOffset conversion path for month_day_nano_interval in arrow_to_pandas by no longer splitting nanoseconds into microseconds + remainder nanoseconds, and updates Python tests to expect nanoseconds-only roundtripping.

Changes:

• Simplify MonthDayNanoIntervalType → Pandas DateOffset conversion to pass interval.nanoseconds directly.
• Update interval/dateoffset-related tests to assert nanoseconds-only behavior.
• Adjust expected DateOffset construction in tests to reflect the new output shape.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
python/pyarrow/src/arrow/python/arrow_to_pandas.cc	Removes microseconds/remainder splitting and passes total nanoseconds into DateOffset.
python/pyarrow/tests/test_pandas.py	Updates roundtrip expectations for DateOffset values produced from month-day-nano intervals.
python/pyarrow/tests/test_array.py	Updates DateOffset expectations in interval array tests to use nanoseconds-only representation.