CI: (deps): Bump xarray from 2025.10.1 to 2026.4.0 in /ci

Bumps [xarray](https://github.com/pydata/xarray) from 2025.10.1 to 2026.4.0.
- [Release notes](https://github.com/pydata/xarray/releases)
- [Commits](pydata/xarray@v2025.10.1...v2026.04.0)

---
updated-dependencies:
- dependency-name: xarray
  dependency-version: 2026.4.0
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

ci/requirements.txt

@@ -6,4 +6,4 @@ pint==0.25.3
 pyproj==3.7.2
 scipy==1.17.1
 traitlets==5.14.3
-xarray==2025.10.1
+xarray==2026.4.0