[HOTFIX] Fix the bug in zhipu api with streaming text (#461)

src/agentscope/models/response.py

@@ -53,7 +53,7 @@ def text(self) -> str:
         if self._text is None:
             if self.stream is not None:
                 for _, chunk in self.stream:
-                    self._text += chunk
+                    self._text = chunk
         return self._text
 
     @text.setter