Clarify or conditionally skip GIS tests when optional GIS dependencies are missing

Nobody has claimed this yet.

While setting up a clean development environment for contributing to mesa-examples, I ran the full test suite locally using:

**python -m pytest**

This resulted in a failure during collection of **test_gis_examples.py** due to missing optional GIS dependencies such as geopandas.

From reviewing the GitHub Actions workflow, it appears that CI runs **test_examples.py** but does not execute **test_gis_examples.py**, which suggests that GIS tests may be considered optional.

Would it make sense to either:

Document that GIS tests require additional optional dependencies, or

Conditionally skip GIS tests when dependencies such as geopandas are not available (for example, using pytest.importorskip)

This could help contributors working on non-GIS examples run the test suite without needing to install the full GIS stack.

Happy to explore a PR if this would be helpful.

Start with test_gis_examples.py and the GitHub Actions workflow, then run python -m pytest in an environment without geopandas. Decide whether the GIS dependency requirement should be documented or the tests conditionally skipped, and verify that the full suite can run for contributors without the optional GIS stack.

Written by the indexing model from the issue text.