Clean up stubtest allowlist and add type stub installation

- Reduce allowlist from ~500 lines to ~170 lines using efficient regex patterns
- Use grouped patterns like (dataarray|dataset|variable) instead of wildcards
- All patterns now match exactly the errors in core modules
- Add step to install type stubs before running stubtest
- Local testing confirms 0 errors, 0 unused patterns

Co-Authored-By: Claude <noreply@anthropic.com>

Author: kkollsga

.github/workflows/ci-additional.yaml

@@ -222,6 +222,10 @@ jobs:
         run: |
           pixi run -e ${{env.PIXI_ENV}} -- python xarray/util/print_versions.py
 
+      - name: Install type stubs
+        run: |
+          pixi run -e ${{env.PIXI_ENV}} -- python -m mypy --install-types --non-interactive xarray/ || true
+
       - name: Run stubtest (core modules)
         run: |
           pixi run -e ${{env.PIXI_ENV}} -- python -m mypy.stubtest \