fix(s3): skip directories before marker in ListObjectVersions pagination (#8890)

* fix(s3): skip directories before marker in ListObjectVersions pagination

ListObjectVersions was re-traversing the entire directory tree from the
beginning on every paginated request, only skipping entries at the leaf
level. For buckets with millions of objects in deep hierarchies, this
caused exponentially slower responses as pagination progressed.

Two optimizations:
1. Use keyMarker to compute a startFrom position at each directory level,
   skipping directly to the relevant entry instead of scanning from the
   beginning (mirroring how ListObjects uses marker descent).
2. Skip recursing into subdirectories whose keys are entirely before the
   keyMarker.

Changes per-page cost from O(entries_before_marker) to O(tree_depth).

* test(s3): add integration test for deep-hierarchy version listing pagination

Adds TestVersioningPaginationDeepDirectoryHierarchy which creates objects
across 20 subdirectories at depth 6 (mimicking Veeam 365 backup layout)
and paginates through them with small maxKeys. Verifies correctness
(no duplicates, sorted order, all objects found) and checks that later
pages don't take dramatically longer than earlier ones — the symptom
of the pre-fix re-traversal bug. Also tests delimiter+pagination
interaction across subdirectories.

* test(s3): strengthen deep-hierarchy pagination assertions

- Replace timing warning (t.Logf) with a failing assertion (t.Errorf)
  so pagination regressions actually fail the test.
- Replace generic count/uniqueness/sort checks on CommonPrefixes with
  exact equality against the expected prefix slice, catching wrong-but-
  sorted results.

* test(s3): use allKeys for exact assertion in deep-hierarchy pagination test

Wire the allKeys slice (previously unused dead code) into the version
listing assertion, replacing generic count/uniqueness/sort checks with
an exact equality comparison against the keys that were created.

test/s3/versioning/s3_versioning_pagination_stress_test.go

@@ -289,6 +289,157 @@ func TestVersioningPaginationMultipleObjectsManyVersions(t *testing.T) {
 	})
 }
 
+// TestVersioningPaginationDeepDirectoryHierarchy tests that paginated ListObjectVersions
+// correctly skips directory subtrees before the key-marker. This reproduces the
+// real-world scenario where Veeam backup objects are spread across many subdirectories
+// (e.g., Mailboxes/<uuid>/ItemsData/<file>) and pagination becomes exponentially
+// slower as the marker advances through the tree.
+//
+// Run with: ENABLE_STRESS_TESTS=true go test -v -run TestVersioningPaginationDeepDirectoryHierarchy -timeout 10m
+func TestVersioningPaginationDeepDirectoryHierarchy(t *testing.T) {
+	if os.Getenv("ENABLE_STRESS_TESTS") != "true" {
+		t.Skip("Skipping stress test. Set ENABLE_STRESS_TESTS=true to run.")
+	}
+
+	client := getS3Client(t)
+	bucketName := getNewBucketName()
+
+	createBucket(t, client, bucketName)
+	defer deleteBucket(t, client, bucketName)
+
+	enableVersioning(t, client, bucketName)
+	checkVersioningStatus(t, client, bucketName, types.BucketVersioningStatusEnabled)
+
+	// Create a deep directory structure mimicking Veeam 365 backup layout:
+	// Backup/Organizations/<org>/Mailboxes/<mailbox>/ItemsData/<file>
+	numMailboxes := 20
+	filesPerMailbox := 5
+	totalObjects := numMailboxes * filesPerMailbox
+	orgPrefix := "Backup/Organizations/org-001/Mailboxes"
+
+	t.Logf("Creating %d objects across %d subdirectories (depth=6)...", totalObjects, numMailboxes)
+	startTime := time.Now()
+
+	allKeys := make([]string, 0, totalObjects)
+	for i := 0; i < numMailboxes; i++ {
+		mailboxId := fmt.Sprintf("mbx-%03d", i)
+		for j := 0; j < filesPerMailbox; j++ {
+			key := fmt.Sprintf("%s/%s/ItemsData/file-%03d.dat", orgPrefix, mailboxId, j)
+			_, err := client.PutObject(context.TODO(), &s3.PutObjectInput{
+				Bucket: aws.String(bucketName),
+				Key:    aws.String(key),
+				Body:   strings.NewReader(fmt.Sprintf("content-%d-%d", i, j)),
+			})
+			require.NoError(t, err)
+			allKeys = append(allKeys, key)
+		}
+	}
+	t.Logf("Created %d objects in %v", totalObjects, time.Since(startTime))
+
+	// Test 1: Paginate through all versions with a broad prefix and small maxKeys.
+	// This forces multiple pages that must skip earlier subdirectory trees.
+	t.Run("PaginateAcrossSubdirectories", func(t *testing.T) {
+		maxKeys := int32(10) // Force many pages to exercise marker skipping
+		var allVersions []types.ObjectVersion
+		var keyMarker, versionIdMarker *string
+		pageCount := 0
+		pageStartTimes := make([]time.Duration, 0)
+
+		for {
+			pageStart := time.Now()
+			resp, err := client.ListObjectVersions(context.TODO(), &s3.ListObjectVersionsInput{
+				Bucket:          aws.String(bucketName),
+				Prefix:          aws.String(orgPrefix + "/"),
+				MaxKeys:         aws.Int32(maxKeys),
+				KeyMarker:       keyMarker,
+				VersionIdMarker: versionIdMarker,
+			})
+			pageDuration := time.Since(pageStart)
+			require.NoError(t, err)
+			pageCount++
+			pageStartTimes = append(pageStartTimes, pageDuration)
+
+			allVersions = append(allVersions, resp.Versions...)
+			t.Logf("Page %d: %d versions in %v (marker: %v)",
+				pageCount, len(resp.Versions), pageDuration,
+				keyMarker)
+
+			if resp.IsTruncated == nil || !*resp.IsTruncated {
+				break
+			}
+			keyMarker = resp.NextKeyMarker
+			versionIdMarker = resp.NextVersionIdMarker
+		}
+
+		assert.Greater(t, pageCount, 1, "Should require multiple pages")
+
+		// Verify listed keys exactly match the keys we created (same elements, same order)
+		listedKeys := make([]string, 0, len(allVersions))
+		for _, v := range allVersions {
+			listedKeys = append(listedKeys, *v.Key)
+		}
+		assert.Equal(t, allKeys, listedKeys,
+			"Listed version keys should exactly match created keys")
+
+		// Check that later pages don't take dramatically longer than earlier ones.
+		// Before the fix, later pages were exponentially slower because they
+		// re-traversed the entire tree. With the fix, all pages should be similar.
+		if len(pageStartTimes) >= 4 {
+			firstQuarter := pageStartTimes[0]
+			lastQuarter := pageStartTimes[len(pageStartTimes)-1]
+			t.Logf("First page: %v, Last page: %v, Ratio: %.1fx",
+				firstQuarter, lastQuarter,
+				float64(lastQuarter)/float64(firstQuarter))
+			// Allow generous 10x ratio to avoid flakiness; before the fix
+			// the ratio was 100x+ on large datasets
+			if lastQuarter > firstQuarter*10 && lastQuarter > 500*time.Millisecond {
+				t.Errorf("Last page took %.1fx longer than first page (%v vs %v) — possible pagination regression",
+					float64(lastQuarter)/float64(firstQuarter), lastQuarter, firstQuarter)
+			}
+		}
+	})
+
+	// Test 2: Paginate with delimiter to verify CommonPrefixes interaction
+	t.Run("PaginateWithDelimiterAcrossSubdirs", func(t *testing.T) {
+		maxKeys := int32(5)
+		var allPrefixes []string
+		var keyMarker *string
+		pageCount := 0
+
+		for {
+			resp, err := client.ListObjectVersions(context.TODO(), &s3.ListObjectVersionsInput{
+				Bucket:    aws.String(bucketName),
+				Prefix:    aws.String(orgPrefix + "/"),
+				Delimiter: aws.String("/"),
+				MaxKeys:   aws.Int32(maxKeys),
+				KeyMarker: keyMarker,
+			})
+			require.NoError(t, err)
+			pageCount++
+
+			for _, cp := range resp.CommonPrefixes {
+				allPrefixes = append(allPrefixes, *cp.Prefix)
+			}
+
+			if resp.IsTruncated == nil || !*resp.IsTruncated {
+				break
+			}
+			keyMarker = resp.NextKeyMarker
+		}
+
+		assert.Greater(t, pageCount, 1, "Should require multiple pages with maxKeys=%d", maxKeys)
+
+		// Build the exact expected prefixes
+		expectedPrefixes := make([]string, 0, numMailboxes)
+		for i := 0; i < numMailboxes; i++ {
+			expectedPrefixes = append(expectedPrefixes,
+				fmt.Sprintf("%s/mbx-%03d/", orgPrefix, i))
+		}
+		assert.Equal(t, expectedPrefixes, allPrefixes,
+			"CommonPrefixes should exactly match expected mailbox prefixes")
+	})
+}
+
 // listAllVersions is a helper to list all versions of a specific object using pagination
 func listAllVersions(t *testing.T, client *s3.Client, bucketName, objectKey string) []types.ObjectVersion {
 	var allVersions []types.ObjectVersion