fix(gateway): 修复 SSE 流式响应 usage 统计错误

message_delta 应完全覆盖 message_start 的 usage 数据，
而非仅在值为 0 时才更新。

backend/internal/service/gateway_service.go

@@ -3372,19 +3372,12 @@ func (s *GatewayService) parseSSEUsage(data string, usage *ClaudeUsage) {
 		} `json:"usage"`
 	}
 	if json.Unmarshal([]byte(data), &msgDelta) == nil && msgDelta.Type == "message_delta" {
-		// output_tokens 总是从 message_delta 获取
+		// message_delta 是推理结束后的最终统计，应完全覆盖 message_start 的数据
+		// 这对于 Claude API 和 GLM 等兼容 API 都是正确的行为
+		usage.InputTokens = msgDelta.Usage.InputTokens
 		usage.OutputTokens = msgDelta.Usage.OutputTokens
-
-		// 如果 message_start 中没有值，则从 message_delta 获取（兼容GLM等API）
-		if usage.InputTokens == 0 {
-			usage.InputTokens = msgDelta.Usage.InputTokens
-		}
-		if usage.CacheCreationInputTokens == 0 {
-			usage.CacheCreationInputTokens = msgDelta.Usage.CacheCreationInputTokens
-		}
-		if usage.CacheReadInputTokens == 0 {
-			usage.CacheReadInputTokens = msgDelta.Usage.CacheReadInputTokens
-		}
+		usage.CacheCreationInputTokens = msgDelta.Usage.CacheCreationInputTokens
+		usage.CacheReadInputTokens = msgDelta.Usage.CacheReadInputTokens
 	}
 }