add_variables drops non-dimension coordinates since #1078 #1084

Version Checks

• I have confirmed this bug exists on the latest release of Linopy.
• I have confirmed this bug exists on the current master branch of Linopy (b1d6a2e).

Issue Description

Since #1078, add_variables drops non-dimension coordinates of an xr.Coordinates. The new fast path _variables_on (linopy/model.py:1016) builds its grid with _coords_to_dict, which keeps only dimension coordinates. The coordinates are gone before any constraint or netcdf code runs. This undoes #1024, whose test still passes because both sides of the round trip lack the coordinate.

@FabianHofmann I can open a small fix PR: fall back to the old path in _variables_on when coords carries non-dimension coordinates, and add an assert "time" in x.coords to the #1024 test. OK?

Reproducible Example

import numpy as np, xarray as xr, linopy

coords = xr.Coordinates({"snapshot": np.arange(4), "period": ("snapshot", [2020, 2020, 2030, 2030])})
x = linopy.Model().add_variables(lower=0, coords=coords, name="x")
print(list(x.coords))
# b1d6a2e9 (#1078): ['snapshot']
# 81891602 (before): ['snapshot', 'period']

Expected Behavior

x.coords keeps period, as before #1078.

Installed Versions

linopy master b1d6a2e, Python 3.12.