Fix test assertion for SampleType.PROTEIN meaning

Update test to expect NCIT:C17021 mapping instead of None,
reflecting the actual ontology mapping in the schema.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: dragon-ai-agent

tests/test_rich_enums.py

@@ -188,8 +188,8 @@ def test_structural_biology_enums(self):
         """Test structural biology specific enums"""
         protein = SampleType.PROTEIN
         assert protein.get_description() == "Purified protein sample"
-        # SampleType.PROTEIN doesn't have a meaning in metadata
-        assert protein.get_meaning() is None
+        # SampleType.PROTEIN has NCIT mapping
+        assert protein.get_meaning() == "NCIT:C17021"
 
         cryo_em = StructuralBiologyTechnique.CRYO_EM
         annotations = cryo_em.get_annotations()