Fix atomic write issue in BatchItemProcessor by making nextExportTime volatile

Co-authored-by: trask <[email protected]>

Author: Copilot

agent/agent-tooling/src/main/java/com/microsoft/applicationinsights/agent/internal/telemetry/BatchItemProcessor.java

@@ -98,7 +98,7 @@ private static final class Worker implements Runnable {
     private final long exporterTimeoutNanos;
     private final int maxPendingExports;
 
-    private long nextExportTime;
+    private volatile long nextExportTime;
 
     private final Queue<TelemetryItem> queue;
     private final int queueCapacity;