code: def test_datetime64_fillna_mismatched_reso_no_rounding(
      self, tz, scalar
  ):
      # GH#56410
      dti = date_range("2016-01-01", periods=3, unit="s", tz=tz)
      item = Timestamp("2016-02-03 04:05:06.789", tz=tz)
      vec = date_range(item, periods=3, unit="ms")
  
      exp_dtype = "M8[ms]" if tz is None else "M8[ms, UTC]"
      expected = Series([item, dti[1], dti[2]], dtype=exp_dtype)
  
      ser = Series(dti)
      ser[0] = NaT
      ser2 = ser.copy()
  
      res = ser.fillna(item)
      res2 = ser2.fillna(Series(vec))
  
      if scalar:
          tm.assert_series_equal(res, expected)
      else:
          tm.assert_series_equal(res2, expected)
  
imports: from datetime import datetime, timedelta, timezone
  import numpy
  import pytest
  import pytz
  from pandas import Categorical, DataFrame, DatetimeIndex, NaT, Period, Series, Timedelta, Timestamp, date_range, isna, timedelta_range
  import pandas._testing
  from pandas.core.arrays import period_array
label_desc: Use image node_id 13 for calling the TestSeriesFillNA obj's underlying member method code with example usage: obj.test_datetime64_fillna_mismatched_reso_no_rounding(tz, scalar) without return types