Fix time masking in sea-ice time series

[xylar]

We need to compute the time mask before we can use it as an index.

Checklist

• Testing comment in the PR documents testing used to verify the changes

[xylar]

Testing

Using zppy, I ran time-series analysis first over years 1985-1990 and then 1985-1995. Whereas I previously saw an error:

Plotting sea-ice area and volume time series...
  Load sea-ice data...
analysis task timeSeriesSeaIceAreaVol failed during run 
Traceback (most recent call last):
  File "/gpfs/fs1/home/ac.xylar/mpas_work/MPAS-Analysis/develop/mpas_analysis/shared/analysis_task.py", line 328, in run
    self.run_task()
    ~~~~~~~~~~~~~^^
  File "/gpfs/fs1/home/ac.xylar/mpas_work/MPAS-Analysis/develop/mpas_analysis/sea_ice/time_series.py", line 259, in run_task
    dsTimeSeries = self._compute_area_vol()
  File "/gpfs/fs1/home/ac.xylar/mpas_work/MPAS-Analysis/develop/mpas_analysis/sea_ice/time_series.py", line 699, in _compute_area_vol
    ds = ds.isel(Time=timeMask)
  File "/gpfs/fs1/home/ac.xylar/chrysalis/miniforge3/envs/mpas_analysis_dev/lib/python3.14/site-packages/xarray/core/dataset.py", line 2848, in isel
    return self._isel_fancy(indexers, drop=drop, missing_dims=missing_dims)
           ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/gpfs/fs1/home/ac.xylar/chrysalis/miniforge3/envs/mpas_analysis_dev/lib/python3.14/site-packages/xarray/core/dataset.py", line 2904, in _isel_fancy
    new_var = var.isel(indexers=var_indexers)
  File "/gpfs/fs1/home/ac.xylar/chrysalis/miniforge3/envs/mpas_analysis_dev/lib/python3.14/site-packages/xarray/core/variable.py", line 1130, in isel
    return self[key]
           ~~~~^^^^^
  File "/gpfs/fs1/home/ac.xylar/chrysalis/miniforge3/envs/mpas_analysis_dev/lib/python3.14/site-packages/xarray/core/variable.py", line 817, in __getitem__
    dims, indexer, new_order = self._broadcast_indexes(key)
                               ~~~~~~~~~~~~~~~~~~~~~~~^^^^^
  File "/gpfs/fs1/home/ac.xylar/chrysalis/miniforge3/envs/mpas_analysis_dev/lib/python3.14/site-packages/xarray/core/variable.py", line 656, in _broadcast_indexes
    self._validate_indexers(key)
    ~~~~~~~~~~~~~~~~~~~~~~~^^^^^
  File "/gpfs/fs1/home/ac.xylar/chrysalis/miniforge3/envs/mpas_analysis_dev/lib/python3.14/site-packages/xarray/core/variable.py", line 709, in _validate_indexers
    raise KeyError(
    ...<4 lines>...
    )
KeyError: 'Indexing with a boolean dask array is not allowed. This will result in a dask array of unknown shape. Such arrays are unsupported by Xarray.Please compute the indexer first using .compute()'

with this fix, the error was not present.

I also successfully ran the developer suite when I also included the fix from #1137. Output can be found here:
https://web.lcrc.anl.gov/public/e3sm/diagnostic_output/xasaydavis/analysis_testing/chrysalis/fix-sea-ice-time-series/