refactor: improve SSE response handling in Playground

- Simplify event listener logic for streaming responses
- Add null-safe checks for payload content
- Optimize message generation and completion flow

web/src/pages/Playground/Playground.js

@@ -157,20 +157,17 @@ const Playground = () => {
       payload: JSON.stringify(payload),
     });
     source.addEventListener("message", (e) => {
-      if (e.data !== "[DONE]") {
-        let payload = JSON.parse(e.data);
-        // console.log("Payload: ", payload);
-        if (payload.choices.length === 0) {
-          source.close();
-          completeMessage();
-        } else {
-          let text = payload.choices[0].delta.content;
-          if (text) {
-            generateMockResponse(text);
-          }
-        }
-      } else {
+      // 只有收到 [DONE] 时才结束
+      if (e.data === "[DONE]") {
+        source.close();
         completeMessage();
+        return;
+      }
+
+      let payload = JSON.parse(e.data);
+      // 检查是否有 delta content
+      if (payload.choices?.[0]?.delta?.content) {
+        generateMockResponse(payload.choices[0].delta.content);
       }
     });