feat: LLM 降级策略 — 全部模型不可用时返回自然语言预设文案

- getFallbackMessage() 随机选择3条预设降级回复
- ProcessInput 失败和流式事件错误均使用降级回复
- 用户看到"开拓者…我现在好像有点恍惚…"而非错误信息

Co-Authored-By: Claude <noreply@anthropic.com>

backend/ai-core/cmd/main.go

@@ -824,6 +824,17 @@ func getEnvInt(key string, fallback int) int {
 	return n
 }
 
+// getFallbackMessage returns a natural-sounding fallback when LLM is unavailable.
+func getFallbackMessage() string {
+	messages := []string{
+		"开拓者…我现在好像有点恍惚…稍等一下好吗？",
+		"唔…忆庭的投影好像出了点小问题。等我一下下♪",
+		"信号有点不太好呢…你稍等我一下哦？",
+	}
+	return messages[time.Now().UnixNano()%int64(len(messages))]
+}
+
+
 // registerPluginTools 从插件实例注册其所有工具到注册中心
 func registerPluginTools(registry *plgManager.ToolRegistry, plugin plgSDK.Plugin) {
 	for _, t := range plugin.Tools() {
@@ -1042,10 +1053,10 @@ func handleChat(
 		BotUID:      req.Source.BotUID,
 	})
 	if err != nil {
-		errData, _ := json.Marshal(map[string]string{"delta": "", "error": fmt.Sprintf("处理失败: %v", err)})
+		fallback := getFallbackMessage()
-		fmt.Fprintf(w, "data: %s\n\n", errData)
+		log.Printf("[chat] ProcessInput 失败，使用降级回复: %v", err)
-		flusher.Flush()
+		fallbackData, _ := json.Marshal(map[string]string{"delta": fallback, "status": "fallback"})
-		fmt.Fprintf(w, "data: [DONE]\n\n")
+		fmt.Fprintf(w, "data: %s\n\n", fallbackData)
 		flusher.Flush()
 		return
 	}
@@ -1071,10 +1082,10 @@ func handleChat(
 
 		case model.StreamError:
 			log.Printf("[chat] 流式错误: %v", event.Error)
-			errData, _ := json.Marshal(map[string]string{"delta": "", "error": event.Error.Error()})
+		fallback := getFallbackMessage()
-			fmt.Fprintf(w, "data: %s\n\n", errData)
+		log.Printf("[chat] 流式事件错误，发送降级回复: %v", event.Error)
-			flusher.Flush()
+		fallbackData, _ := json.Marshal(map[string]string{"delta": fallback, "status": "fallback"})
-			fmt.Fprintf(w, "data: [DONE]\n\n")
+		fmt.Fprintf(w, "data: %s\n\n", fallbackData)
 			flusher.Flush()
 			return