Get generic entity grid with configurable associations

Returns a generic entity × entity grid with configurable association categories.

This endpoint allows you to specify:
- **column_association_category**: One or more association categories for the
  context → column hop (e.g., causal gene-disease, correlated gene-disease)
- **row_association_category**: The association category for the column → row hop
- **row_grouping**: How to group rows (histopheno for body systems, or none)
- **group_columns_by_category**: Whether to sort columns by their source association

Example: To get a grid of diseases (causal + correlated) and their phenotypes for a gene:
```
GET /entity-grid/HGNC:4851?column_association_category=biolink:CausalGeneToDiseaseAssociation
    &column_association_category=biolink:CorrelatedGeneToDiseaseAssociation
    &row_association_category=biolink:DiseaseToPhenotypicFeatureAssociation
    &group_columns_by_category=true
```

The response includes source_association_category on each column entity,
allowing the frontend to display different column groups.