fix: pin xarray below 2026.3 for linopy compatibility

xarray 2026.3+ breaks the Dataset constructor in ways linopy still relies
on, causing TypeError: "Passing a Dataset as data_vars to the Dataset
constructor is not supported" across the test suite.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: FBumann

pyproject.toml

@@ -34,7 +34,7 @@ dependencies = [
     # Core scientific computing
     "numpy >= 1.21.5, < 3",
     "pandas >= 2.0.0, < 3",
-    "xarray >=2024.2.0, <2026.5", # CalVer: allow through next calendar year
+    "xarray >=2024.2.0, <2026.3", # TODO: revert to <2026.5 once linopy releases xarray 2026.3+ compat fix
     # Optimization and data handling
     "linopy >=0.5.1, <0.7", # Widened from patch pin to minor range
     "netcdf4 >=1.6.1, <1.7.5",  # 1.7.4 missing wheels, revert to < 2 later