fix: seed trace server counts from ActiveTopology to match detection logic

The decision trace was building serverVolumeCounts only from metrics,
missing zero-volume servers seeded from ActiveTopology by Detection.
This could cause the trace to report wrong server counts, incorrect
imbalance ratios, or spurious "too few servers" messages. Pass
activeTopology into the trace function and seed server counts the
same way Detection does.

weed/plugin/worker/volume_balance_handler.go

@@ -230,7 +230,7 @@ func (h *VolumeBalanceHandler) Detect(
 		return err
 	}
 
-	if traceErr := emitVolumeBalanceDetectionDecisionTrace(sender, metrics, workerConfig.TaskConfig, results); traceErr != nil {
+	if traceErr := emitVolumeBalanceDetectionDecisionTrace(sender, metrics, activeTopology, workerConfig.TaskConfig, results); traceErr != nil {
 		glog.Warningf("Plugin worker failed to emit volume_balance detection trace: %v", traceErr)
 	}
 
@@ -262,6 +262,7 @@ func (h *VolumeBalanceHandler) Detect(
 func emitVolumeBalanceDetectionDecisionTrace(
 	sender DetectionSender,
 	metrics []*workertypes.VolumeHealthMetrics,
+	activeTopology *topology.ActiveTopology,
 	taskConfig *balancetask.Config,
 	results []*workertypes.TaskDetectionResult,
 ) error {
@@ -357,7 +358,25 @@ func emitVolumeBalanceDetectionDecisionTrace(
 			continue
 		}
 
+		// Seed server counts from topology so zero-volume servers are included,
+		// matching the same logic used in balancetask.Detection.
 		serverVolumeCounts := make(map[string]int)
+		if activeTopology != nil {
+			topologyInfo := activeTopology.GetTopologyInfo()
+			if topologyInfo != nil {
+				for _, dc := range topologyInfo.DataCenterInfos {
+					for _, rack := range dc.RackInfos {
+						for _, node := range rack.DataNodeInfos {
+							for diskTypeName := range node.DiskInfos {
+								if diskTypeName == diskType {
+									serverVolumeCounts[node.Id] = 0
+								}
+							}
+						}
+					}
+				}
+			}
+		}
 		for _, metric := range diskMetrics {
 			serverVolumeCounts[metric.Server]++
 		}

weed/plugin/worker/volume_balance_handler_test.go

@@ -228,7 +228,7 @@ func TestEmitVolumeBalanceDetectionDecisionTraceNoTasks(t *testing.T) {
 		{VolumeID: 4, Server: "server-b", DiskType: "hdd"},
 	}
 
-	if err := emitVolumeBalanceDetectionDecisionTrace(sender, metrics, config, nil); err != nil {
+	if err := emitVolumeBalanceDetectionDecisionTrace(sender, metrics, nil, config, nil); err != nil {
 		t.Fatalf("emitVolumeBalanceDetectionDecisionTrace error: %v", err)
 	}
 	if len(sender.events) < 2 {