Insulin is split between chemical and protein

[gaurav]

In Babel 2025nov4, we have UMLS:C0021641 "Insulin" (biolink:Protein) distinct from CHEBI:5931 "Insulin human" (biolink:ChemicalEntity).

Our options are:

1. Combine biolink:Proteins into drugs in the existing DrugChemical conflate.
2. Create a new DrugProtein conflation to handle these cases (A possible solution to the protein/chemical overlap problem (DrugProtein conflation) #440)