fix(datasource): 类型注解使用字符串前向引用避免运行时错误

datasource/ccxt_source.py

@@ -117,7 +117,7 @@ class CCXTSource:
             self._proxy_started = False
             print("socks2http 代理已停止")
     
-    def _create_exchange(self) -> Optional[ccxt.Exchange]:
+    def _create_exchange(self) -> Optional['ccxt.Exchange']:
         """创建交易所实例"""
         if not CCXT_AVAILABLE:
             return None