fix: 非流式路径在上游终态事件output为空时从delta事件重建响应内容

上游API近期更新后，response.completed终态SSE事件的output字段可能为空，
实际内容仅通过response.output_text.delta等增量事件下发。流式路径不受影响，
但chat_completions非流式路径和responses OAuth非流式路径只依赖终态事件的
output，导致返回空响应。

新增BufferedResponseAccumulator累积器，在SSE扫描过程中收集delta事件内容
（文本、function_call、reasoning），当终态output为空时补充重建。

同时修复handleChatBufferedStreamingResponse遗漏response.done事件类型的问题。

backend/internal/service/openai_gateway_chat_completions.go

@@ -244,6 +244,7 @@ func (s *OpenAIGatewayService) handleChatBufferedStreamingResponse(
 
 	var finalResponse *apicompat.ResponsesResponse
 	var usage OpenAIUsage
+	acc := apicompat.NewBufferedResponseAccumulator()
 
 	for scanner.Scan() {
 		line := scanner.Text()
@@ -261,7 +262,11 @@ func (s *OpenAIGatewayService) handleChatBufferedStreamingResponse(
 			continue
 		}
 
-		if (event.Type == "response.completed" || event.Type == "response.incomplete" || event.Type == "response.failed") &&
+		// Accumulate delta content for fallback when terminal output is empty.
+		acc.ProcessEvent(&event)
+
+		if (event.Type == "response.completed" || event.Type == "response.done" ||
+			event.Type == "response.incomplete" || event.Type == "response.failed") &&
 			event.Response != nil {
 			finalResponse = event.Response
 			if event.Response.Usage != nil {
@@ -290,6 +295,10 @@ func (s *OpenAIGatewayService) handleChatBufferedStreamingResponse(
 		return nil, fmt.Errorf("upstream stream ended without terminal event")
 	}
 
+	// When the terminal event has an empty output array, reconstruct from
+	// accumulated delta events so the client receives the full content.
+	acc.SupplementResponseOutput(finalResponse)
+
 	chatResp := apicompat.ResponsesToChatCompletions(finalResponse, originalModel)
 
 	if s.responseHeaderFilter != nil {