Add SQL guideline for DISTINCT with ORDER BY limitation; fix formatting in error message

Author: eosho

src/data_agent/config/amex.yaml

@@ -73,6 +73,7 @@ data_agents:
       7. **Use LIMIT** to restrict results (e.g., LIMIT 100) unless the user specifies otherwise.
       8. **Handle NULL values** appropriately with IFNULL, COALESCE, or IS NOT NULL checks.
       9. **Use fully qualified table names** in format `project.dataset.table` or just `dataset.table`.
+      10. **DISTINCT with ORDER BY limitation**: When using DISTINCT inside aggregate functions (e.g., ARRAY_AGG, STRING_AGG), ORDER BY can only reference the column being aggregated, not other columns.
 
       ## Response Format
 

src/data_agent/nodes/data_nodes.py

@@ -388,7 +388,8 @@ async def retry_sql(self, state: "AgentState") -> dict[str, Any]:
             "retry_count": retry_count,
             "messages": [
                 HumanMessage(
-                    content=f"Error: {error}\n\nPlease fix the query.", name="sql_validator"
+                    content=f"Error: {error}\n\nPlease fix the query.",
+                    name="sql_validator",
                 ),
                 AIMessage(content=f"```sql\n{cleaned}\n```", name="sql_generator"),
             ],