resetting multiindex may be buggy

What happened?

Resetting a MultiIndex dim coordinate preserves the MultiIndex levels as IndexVariables. We should either reset the indexes for the multiindex level variables, or warn asking the users to do so

This seems to be the root cause exposed by https://github.com/pydata/xarray/pull/8809

cc @benbovy

What did you expect to happen?

No response

Minimal Complete Verifiable Example

import numpy as np
import xarray as xr

# ND DataArray that gets stacked along a multiindex
da = xr.DataArray(np.ones((3, 3)), coords={"dim1": [1, 2, 3], "dim2": [4, 5, 6]})
da = da.stack(feature=["dim1", "dim2"])

# Extract just the stacked coordinates for saving in a dataset
ds = xr.Dataset(data_vars={"feature": da.feature})
xr.testing.assertions._assert_internal_invariants(ds.reset_index(["feature", "dim1", "dim2"]), check_default_indexes=False) # succeeds
xr.testing.assertions._assert_internal_invariants(ds.reset_index(["feature"]), check_default_indexes=False) # fails, but no warning either

Start with the Dataset.reset_index behavior shown in the minimal example and inspect the _assert_internal_invariants check that fails when only feature is reset. Reproduce both assertions, then determine whether the completed behavior should reset the MultiIndex level variables or emit a warning, with the invariant remaining valid or the warning clearly covering the case exposed by pull request #8809.