fix(autotest): skip export tests that need geopandas

[jdhughes-dev]

The nightly optional dependency job installs the optional dependencies and then removes three of them chosen by the date, so it reaches combinations the job that installs none of them does not. Shapefile export goes through geopandas, but the export tests were marked as needing only pyshp and failed with an ImportError rather than being skipped on the night geopandas was removed. The two export tests that do not go through geopandas keep their existing mark.

The mark for sklearn on test_save_load_node_mapping_structured was added commented out and never took effect, so that test failed the same way when scikit-learn was removed.

Reproducing the 2026-08-07 run of that job locally (shapely, fiona and scikit-learn removed) takes it from 37 dependency failures to none. The one failure left in that configuration is test_vtk_vector, which is a separate defect fixed in #2799.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.7%. Comparing base (556c088) to head (565843e).
⚠️ Report is 191 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2800      +/-   ##
===========================================
+ Coverage     55.5%    72.7%   +17.2%     
===========================================
  Files          644      662      +18     
  Lines       124135   133024    +8889     
===========================================
+ Hits         68947    96821   +27874     
+ Misses       55188    36203   -18985     

see 583 files with indirect coverage changes

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