Function deeply_nested_embedded_schema 

pub async fn deeply_nested_embedded_schema(test: &mut Test)

Tests deeply nested embedded types (3+ levels) to verify schema building handles arbitrary nesting depth correctly. Validates:

• App schema: all embedded models registered
• DB schema: deeply nested fields flattened with proper prefixes
• Mapping: nested Field::Struct structure with correct columns maps
• model_to_table: nested projection expressions