GH-48654: [Python] Test timestamp from int without pandas dependency

[HyukjinKwon]

Rationale for this change

The test test_sequence_timestamp_from_int_with_unit() was marked with
@pytest.mark.pandas, meaning it was skipped when pandas was not installed.
This left the non-pandas code path untested.

Specifically in order to test:

arrow/python/pyarrow/scalar.pxi

Lines 652 to 661 in 744f0ec

	if _pandas_api.have_pandas:
	return _pandas_api.pd.Timestamp(value, tz=tzinfo, unit='ns')
	# otherwise safely truncate to microsecond resolution datetime
	if value % 1000 != 0:
	raise ValueError(
	f"Nanosecond resolution temporal type {value} is not safely "
	"convertible to microseconds to convert to datetime.datetime. "
	"Install pandas to return as Timestamp with nanosecond "
	"support or access the .value attribute."
	)

The TODO was introduced in commit 286bf7c when making pandas
an optional dependency:

arrow/python/pyarrow/tests/test_convert_builtin.py

Lines 731 to 732 in 286bf7c

	# TODO(wesm): This test might be rewritten to assert the actual behavior
	# when pandas is not installed

What changes are included in this PR?

Rewrote the test to work both with and without pandas by:

1. Removed @pytest.mark.pandas decorator
2. Removed TODO comment left and added conditional testing for nanosecond timestamp behavior:
   • With pandas: Validates pd.Timestamp with full nanosecond precision
   • Without pandas: Validates that as_py() raises ValueError for values
     not safely convertible to microseconds

Are these changes tested?

Yes. I manually tested with/without pandas:

conda remove pandas
pytest pyarrow/tests/test_convert_builtin.py -k "timestamp_from_int_with_unit" -xvs
conda install pandas==2.3.3
pytest pyarrow/tests/test_convert_builtin.py -k "timestamp_from_int_with_unit" -xvs

Are there any user-facing changes?

No, test-only.

• GitHub Issue: [Python] Test timestamp from int without pandas dependency #48654

[github-actions]

⚠️ GitHub issue #48654 has been automatically assigned in GitHub to PR creator.