Fix reindex on Dataset from MultiIndex DataFrame with RuntimeError

[csernazs]

• Closes reindex fails on Dataset from MultiIndex DataFrame with RuntimeError #10347
• Tests added
• User visible changes (including notable bug fixes) are documented in whats-new.rst
• New functions/methods are listed in api.rst

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[benbovy]

Thanks @csernazs!

Just added a suggestion for the test, otherwise it looks good.

[benbovy]

A simpler test for the fixed issue would something like:

def test_reindex_with_multiindex_level(self) -> None:
    # test for https://github.com/pydata/xarray/issues/10347
    mindex = pd.MultiIndex.from_product([[100, 200, 300], [1, 2, 3, 4]], names=["x", "y"])
    y_idx = PandasIndex(mindex.levels[1], "y")

    ds1 = xr.Dataset(coords={"y": [1, 2, 3]})
    ds2 = xr.Dataset(coords=xr.Coordinates.from_xindex(y_idx))

    actual = ds1.reindex(y=ds2.y)
    assert_identical(actual, ds2)

I'd also move this test into test_dataset.py and leave test_indexes.py for testing the logic that is implemented in xarray.core.indexes.

[csernazs]

Thank you @benbovy for the review!

I have implemented the test as you suggested. I put the test to the end of the file, if that's not correct, let me know.

Zsolt