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_service.go

@@ -21,6 +21,7 @@ import (
 	"time"
 
 	"github.com/Wei-Shaw/sub2api/internal/config"
+	"github.com/Wei-Shaw/sub2api/internal/pkg/apicompat"
 	"github.com/Wei-Shaw/sub2api/internal/pkg/logger"
 	"github.com/Wei-Shaw/sub2api/internal/pkg/openai"
 	"github.com/Wei-Shaw/sub2api/internal/util/responseheaders"
@@ -3901,6 +3902,16 @@ func (s *OpenAIGatewayService) handleOAuthSSEToJSON(resp *http.Response, c *gin.
 		if parsedUsage, parsed := extractOpenAIUsageFromJSONBytes(finalResponse); parsed {
 			*usage = parsedUsage
 		}
+		// When the terminal event has an empty output array, reconstruct
+		// output from accumulated delta events so the client gets full content.
+		// gjson Array() returns empty slice for null, missing, or empty arrays.
+		if len(gjson.GetBytes(finalResponse, "output").Array()) == 0 {
+			if outputJSON, reconstructed := reconstructResponseOutputFromSSE(bodyText); reconstructed {
+				if patched, err := sjson.SetRawBytes(finalResponse, "output", outputJSON); err == nil {
+					finalResponse = patched
+				}
+			}
+		}
 		body = finalResponse
 		if originalModel != mappedModel {
 			body = s.replaceModelInResponseBody(body, mappedModel, originalModel)
@@ -4002,6 +4013,34 @@ func extractCodexFinalResponse(body string) ([]byte, bool) {
 	return nil, false
 }
 
+// reconstructResponseOutputFromSSE scans raw SSE body text for delta events and
+// returns a JSON-encoded output array reconstructed from accumulated deltas.
+// Returns (nil, false) if no content was found in deltas.
+func reconstructResponseOutputFromSSE(bodyText string) ([]byte, bool) {
+	acc := apicompat.NewBufferedResponseAccumulator()
+	lines := strings.Split(bodyText, "\n")
+	for _, line := range lines {
+		data, ok := extractOpenAISSEDataLine(line)
+		if !ok || data == "" || data == "[DONE]" {
+			continue
+		}
+		var event apicompat.ResponsesStreamEvent
+		if err := json.Unmarshal([]byte(data), &event); err != nil {
+			continue
+		}
+		acc.ProcessEvent(&event)
+	}
+	if !acc.HasContent() {
+		return nil, false
+	}
+	output := acc.BuildOutput()
+	outputJSON, err := json.Marshal(output)
+	if err != nil {
+		return nil, false
+	}
+	return outputJSON, true
+}
+
 func (s *OpenAIGatewayService) parseSSEUsageFromBody(body string) *OpenAIUsage {
 	usage := &OpenAIUsage{}
 	lines := strings.Split(body, "\n")