Inconsistent multiplication behavior: Variable vs LinearExpression with subset coordinates

[FabianHofmann]

Issue Description

Multiplying with a constant that has a subset of coordinates on a shared dimension behaves differently for Variable vs LinearExpression:

• x * subset_da works (produces NaN coefficients)
• (1 * x) * subset_da fails with AssertionError

Mathematically these should be equivalent.

Reproducible Example

import pandas as pd
import xarray as xr
import linopy

m = linopy.Model()
x = m.add_variables(coords=[pd.Index(range(5), name="i")])

subset_da = xr.DataArray([10.0, 30.0], dims=["i"], coords={"i": [1, 3]})

# This works
result1 = x * subset_da
print(result1.coeffs.values)  # [nan, 10., nan, 30., nan]

# This fails with AssertionError
expr = 1 * x
result2 = expr * subset_da  # AssertionError in _multiply_by_constant

Expected Behavior

Both operations should behave consistently. Either:

1. Both succeed with same result, or
2. Both raise an error indicating misaligned coordinates

Installed Versions

Details

master branch (commit 59f92ae)