Add regression test and fix for missing offsetString in EventContext

Added EventContextTest.updateCheckpointAsyncSetsOffsetString() which verifies
that offsetString is populated on the Checkpoint passed to the store. This test
fails without the fix (offsetString is null) and passes with it.

Fix: added setOffsetString(eventData.getOffsetString()) in
EventContext.updateCheckpointAsync(), matching 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);
     }

sdk/eventhubs/azure-messaging-eventhubs/src/test/java/com/azure/messaging/eventhubs/models/EventContextTest.java

@@ -0,0 +1,76 @@
+// Copyright (c) Microsoft Corporation. All rights reserved.
+// Licensed under the MIT License.
+
+package com.azure.messaging.eventhubs.models;
+
+import com.azure.messaging.eventhubs.CheckpointStore;
+import com.azure.messaging.eventhubs.EventData;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+import org.mockito.ArgumentCaptor;
+import org.mockito.Mock;
+import org.mockito.MockitoAnnotations;
+import reactor.core.publisher.Mono;
+import reactor.test.StepVerifier;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNotNull;
+import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.Mockito.verify;
+import static org.mockito.Mockito.when;
+
+/**
+ * Tests for {@link EventContext}.
+ */
+class EventContextTest {
+    private final PartitionContext partitionContext
+        = new PartitionContext("TEST_NAMESPACE", "TEST_EVENT_HUB", "TEST_DEFAULT_GROUP", "TEST_PARTITION_ID");
+
+    @Mock
+    private CheckpointStore checkpointStore;
+    @Mock
+    private EventData eventData;
+
+    @BeforeEach
+    void beforeEach() {
+        MockitoAnnotations.initMocks(this);
+    }
+
+    /**
+     * Verifies that updateCheckpointAsync sets offsetString on the checkpoint.
+     * Regression test for https://github.com/Azure/azure-sdk-for-java/issues/46752
+     * where only setOffset(Long) was called, causing BlobCheckpointStore to store
+     * null offset because it reads getOffsetString().
+     */
+    @Test
+    void updateCheckpointAsyncSetsOffsetString() {
+        // Arrange
+        final Long sequenceNumber = 10L;
+        final Long offset = 15L;
+        final String offsetString = "15";
+
+        when(eventData.getSequenceNumber()).thenReturn(sequenceNumber);
+        when(eventData.getOffset()).thenReturn(offset);
+        when(eventData.getOffsetString()).thenReturn(offsetString);
+        when(checkpointStore.updateCheckpoint(any(Checkpoint.class))).thenReturn(Mono.empty());
+
+        final EventContext context = new EventContext(partitionContext, eventData, checkpointStore, null);
+
+        // Act
+        StepVerifier.create(context.updateCheckpointAsync()).verifyComplete();
+
+        // Assert - offsetString must be set on the checkpoint passed to the store
+        ArgumentCaptor<Checkpoint> captor = ArgumentCaptor.forClass(Checkpoint.class);
+        verify(checkpointStore).updateCheckpoint(captor.capture());
+
+        Checkpoint captured = captor.getValue();
+        assertEquals(partitionContext.getFullyQualifiedNamespace(), captured.getFullyQualifiedNamespace());
+        assertEquals(partitionContext.getEventHubName(), captured.getEventHubName());
+        assertEquals(partitionContext.getConsumerGroup(), captured.getConsumerGroup());
+        assertEquals(partitionContext.getPartitionId(), captured.getPartitionId());
+        assertEquals(sequenceNumber, captured.getSequenceNumber());
+        assertEquals(offset, captured.getOffset());
+        assertNotNull(captured.getOffsetString(), "offsetString must not be null - BlobCheckpointStore depends on it");
+        assertEquals(offsetString, captured.getOffsetString());
+    }
+}