Get traversable associations for an entity type

Returns association types that can be traversed from a given entity category.

For each association, returns:
- **category**: The biolink association category
- **label**: Human-readable label for UI display
- **context_field**: Which field ("subject" or "object") the context entity occupies
- **target_category**: The biolink category of entities at the other end

This enables dynamic UI that shows only valid association options based on
the selected entity type, supporting bidirectional traversal of associations.

Example: For biolink:Gene, this returns gene-disease, gene-phenotype, gene-homology, etc.
For biolink:Disease, this returns disease-phenotype, and also gene-disease (reverse direction).