Fix missing offsetString in EventContext checkpoint update

EventContext.updateCheckpointAsync() only called setOffset(Long) (deprecated),
but BlobCheckpointStore.updateCheckpoint() reads getOffsetString() which returned
null - causing offset to never be stored in the checkpoint blob metadata.

Added setOffsetString(eventData.getOffsetString()) to match the pattern already
used in EventBatchContext.

Fixes #46752

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: Johnathan Walker

sdk/eventhubs/azure-messaging-eventhubs/src/main/java/com/azure/messaging/eventhubs/models/EventContext.java

@@ -94,6 +94,7 @@ public Mono<Void> updateCheckpointAsync() {
                 .setConsumerGroup(partitionContext.getConsumerGroup())
                 .setPartitionId(partitionContext.getPartitionId())
                 .setSequenceNumber(eventData.getSequenceNumber())
+                .setOffsetString(eventData.getOffsetString())
                 .setOffset(eventData.getOffset());
         return this.checkpointStore.updateCheckpoint(checkpoint);
     }