fix: restore droppedEventCount property on SegmentDestination

Property declaration was lost during branch rebase.

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

Author: abueide

packages/core/src/plugins/SegmentDestination.ts

@@ -40,6 +40,7 @@ type ErrorAggregation = {
 export class SegmentDestination extends DestinationPlugin {
   type = PluginType.destination;
   key = SEGMENT_DESTINATION_KEY;
+  droppedEventCount = 0;
   private apiHost?: string;
   private httpConfig?: HttpConfig;
   private settingsResolve: () => void;