fix: increment droppedEventCount at SegmentDestination drop sites

Wire up the droppedEventCount counter (added in cli-flush-retry-loop)
at the two places SegmentDestination permanently removes events:
permanent errors and retry limit exceeded.

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

Author: abueide

packages/core/src/plugins/SegmentDestination.ts

@@ -272,6 +272,7 @@ export class SegmentDestination extends DestinationPlugin {
       await this.queuePlugin.dequeueByMessageIds(
         aggregation.permanentErrorMessageIds
       );
+      this.droppedEventCount += aggregation.permanentErrorMessageIds.length;
       this.analytics?.logger.error(
         `Dropped ${aggregation.permanentErrorMessageIds.length} events due to permanent errors`
       );
@@ -281,6 +282,7 @@ export class SegmentDestination extends DestinationPlugin {
       await this.queuePlugin.dequeueByMessageIds(
         aggregation.retryableMessageIds
       );
+      this.droppedEventCount += aggregation.retryableMessageIds.length;
       this.analytics?.logger.error(
         `Dropped ${aggregation.retryableMessageIds.length} events due to retry limit exceeded`
       );