fix: Correct old_filename reference in update_filename_success test

Author: reddevilmidzy

tests/file_manager/handlers.rs

@@ -403,9 +403,8 @@ async fn update_filename_success() {
     // Modify file
     let client = reqwest::Client::new();
     let new_filename = "aplusb-AC.py";
-    let old_filename = filename;
     let update_data = serde_json::json!({
-        "old_filename": old_filename,
+        "old_filename": filename,
         "new_filename": new_filename
     });