store: Fix missing struct_field_mappings field in test mock_abi

Add missing struct_field_mappings field to MappingABI initialization
in store test to fix compilation error.

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

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

Author: fubhy

store/test-store/tests/postgres/store.rs

@@ -1074,6 +1074,7 @@ fn mock_abi() -> MappingABI {
             .as_bytes(),
         )
         .unwrap(),
+        struct_field_mappings: std::collections::HashMap::new(),
     }
 }