fix: make assert_linequal term-order invariant

[FBumann]

Summary

• assert_linequal now sorts both sides by variable labels along _term before comparing, so expressions with different term orderings are correctly recognized as semantically equal.
• Fixes false failures when comparing constraints that went through the CSR round-trip (freeze_constraints=True) against freshly built expressions, since csr.sum_duplicates() sorts terms by variable label.

Reproducer (from #630 comment)

from linopy import Model
from linopy.testing import assert_linequal
import pandas as pd

m = Model(freeze_constraints=True)
coords = pd.RangeIndex(3, name="time")
x = m.add_variables(coords=[coords], name="x")
y = m.add_variables(coords=[coords], name="y")

m.add_constraints(y - x == 0, name="con")

desired = y == x
assert_linequal(m.constraints["con"].lhs, desired.lhs)  # was failing, now passes

Test plan

• Reproducer passes
• All 2665 existing tests pass
• Linter and pre-commit hooks pass

Replaces #639 (was opened from fork, CI didn't trigger fully).

🤖 Generated with Claude Code

[FBumann]

Closed in favor of #639