Make GNM data validation diagnostics deterministic

[HosniBelfeki]

Summary

• Return sorted missing and extra field diagnostics from GNM model data validation.
• Add a focused regression test covering deterministic validation output.

Tests

• python -m pytest gnm\shape\gnm_data_loader_test.py -q
• python -m pylint gnm\shape\gnm_data_loader.py gnm\shape\gnm_data_loader_test.py

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