fix(service): 修复扣费结果类型处理

- service/agent/billing.rs: 适配新的 BillingResult 枚举类型
- 将 InsufficientBalance 错误转换为 AppError::BadRequest

libs/service/agent/billing.rs

@@ -12,13 +12,21 @@ impl AppService {
         input_tokens: i64,
         output_tokens: i64,
     ) -> Result<agent::billing::BillingRecord, AppError> {
-        Ok(agent::billing::record_ai_usage(
+        use agent::billing::BillingResult;
+
+        match agent::billing::record_ai_usage(
             &self.db,
             project_uid,
             model_id,
             input_tokens,
             output_tokens,
         )
-        .await?)
+        .await?
+        {
+            BillingResult::Success(record) => Ok(record),
+            BillingResult::InsufficientBalance { message } => {
+                Err(AppError::BadRequest(message))
+            }
+        }
     }
 }