perf(service): SSE Scanner buffer 改用 sync.Pool 复用，减少高并发 GC 压力

将流式响应中 bufio.Scanner 的 64KB buffer 从每次 make 分配改为
sync.Pool 复用，统一切片表达式为 [:0]、变量命名为 scanBuf，
并补充对应的单元测试。

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/internal/service/gateway_service.go

@@ -4031,7 +4031,8 @@ func (s *GatewayService) handleStreamingResponse(ctx context.Context, resp *http
 	if s.cfg != nil && s.cfg.Gateway.MaxLineSize > 0 {
 		maxLineSize = s.cfg.Gateway.MaxLineSize
 	}
-	scanner.Buffer(make([]byte, 64*1024), maxLineSize)
+	scanBuf := getSSEScannerBuf64K()
+	scanner.Buffer(scanBuf[:0], maxLineSize)
 
 	type scanEvent struct {
 		line string
@@ -4050,7 +4051,8 @@ func (s *GatewayService) handleStreamingResponse(ctx context.Context, resp *http
 	}
 	var lastReadAt int64
 	atomic.StoreInt64(&lastReadAt, time.Now().UnixNano())
-	go func() {
+	go func(scanBuf *sseScannerBuf64K) {
+		defer putSSEScannerBuf64K(scanBuf)
 		defer close(events)
 		for scanner.Scan() {
 			atomic.StoreInt64(&lastReadAt, time.Now().UnixNano())
@@ -4061,7 +4063,7 @@ func (s *GatewayService) handleStreamingResponse(ctx context.Context, resp *http
 		if err := scanner.Err(); err != nil {
 			_ = sendEvent(scanEvent{err: err})
 		}
-	}()
+	}(scanBuf)
 	defer close(done)
 
 	streamInterval := time.Duration(0)