fix(dameng): deduplicate column metadata

crates/dbx-web/src/routes/connection.rs

@@ -209,6 +209,7 @@ mod tests {
             sse_channels: RwLock::new(HashMap::new()),
             sql_file_executions: RwLock::new(HashMap::new()),
             login_rate_limit: Mutex::new(LoginRateLimit { fail_count: 0, locked_until: None }),
+            export_files: RwLock::new(HashMap::new()),
         });
         (state, dir)
     }