优化代码健壮性，解决部分硬编码问题

sf_mrs_connect/models/ftp_operate.py

@@ -21,6 +21,16 @@ class FtpController():
         except Exception:
             logging.info("ftp连接失败")
 
+    def file_exists(self, path):
+        # 检查文件是否存在于FTP服务器上
+        try:
+            self.ftp.cwd(os.path.dirname(path))
+            files = self.ftp.nlst()
+            return os.path.basename(path) in files
+        except Exception as e:
+            logging.error(f"Error checking file: {e}")
+            return False
+
     # 下载目录下的文件
     def download_file_tree(self, target_dir, serverdir):
         if not os.path.exists(serverdir):