Data published from xcube server where calculations on the fly add new data variables raises error for mldatasets

[konstntokas]

Describe the bug
When using xcube-stac to open data published by xcube server calculations on the fly add new data variables raises an error, if mldataset is desired. xcube-stac renames variables to distinguish data variables of different assets sharing the same name. When calculations are performed on a dataset where data variables are added, the renaming dictionary picks those names. However when actually accessing the data source with mlds.get_dataset(idx_level), the original dataset is opened, which does not have these data variable, resulting in an error.

To Reproduce
Steps to reproduce the behavior:

1. run xcube server demo example: xcube serve -vvv -c examples/serve/demo/config.yml
2. use xcube-stac to open data:

from xcube.core.store import new_data_store

url = "http://127.0.0.1:8080/ogc"
store = new_data_store("stac", url=url)
mlds = store.open_data("collections/datacubes/items/local", data_type="mldataset")
ds = mlds.get_dataset(2)
print(ds)

Returned error:

Traceback (most recent call last):
  File "/home/konstantin/bc_kon/01_coding/00_testing/error_xcube_server.py", line 6, in <module>
    ds = mlds.get_dataset(2)
         ^^^^^^^^^^^^^^^^^^^
  File "/home/konstantin/micromamba/envs/xcube-stac/lib/python3.12/site-packages/xcube/core/mldataset/lazy.py", line 92, in get_dataset
    level_dataset = self._get_dataset_lazily(index, self._parameters)
                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/konstantin/micromamba/envs/xcube-stac/lib/python3.12/site-packages/xcube/core/mldataset/mapped.py", line 32, in _get_dataset_lazily
    return self._mapper_function(
           ^^^^^^^^^^^^^^^^^^^^^^
  File "/home/konstantin/bc_kon/01_coding/01_github/xcube-stac/xcube_stac/utils.py", line 508, in _xarray_rename_vars
    return ds.rename_vars(name_dict)
           ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/konstantin/micromamba/envs/xcube-stac/lib/python3.12/site-packages/xarray/core/dataset.py", line 4396, in rename_vars
    raise ValueError(
ValueError: cannot rename 'lat_bnds' because it is not a variable or coordinate in this dataset

Expected behavior
Levels of multi-level datasets shall be opened without error.