test(s3/lifecycle): integration coverage for multi-bucket sweep

A single shell-driven shard sweep must process every bucket carrying
lifecycle config, not just the first one alphabetically. Pinned
because the scheduler iterates the buckets directory and a regression
that returns early after the first match would silently disable
lifecycle for every later bucket.

Two buckets, each with their own prefix-expiration rule and a
backdated object. Both must be expired after the same sweep.

test/s3/lifecycle/s3_lifecycle_multi_bucket_test.go

@@ -0,0 +1,61 @@
+// Multiple-bucket integration scenario.
+package lifecycle
+
+import (
+	"context"
+	"testing"
+	"time"
+
+	"github.com/aws/aws-sdk-go-v2/aws"
+	"github.com/aws/aws-sdk-go-v2/service/s3"
+	"github.com/stretchr/testify/require"
+)
+
+// TestLifecycleMultipleBucketsInOneSweep: a single shell-driven shard
+// sweep must process every bucket carrying lifecycle config, not just
+// the first one alphabetically. Pinned because the scheduler iterates
+// the buckets directory and a regression that returns early after the
+// first match would silently disable lifecycle for every later bucket.
+//
+// Two buckets, each with its own 1-day prefix-expiration rule and one
+// backdated object. After the worker runs, both objects must be gone.
+func TestLifecycleMultipleBucketsInOneSweep(t *testing.T) {
+	c := s3Client(t)
+	fc, fcClose := filerClient(t)
+	defer fcClose()
+
+	bucketA := uniqueBucket("multi-a")
+	bucketB := uniqueBucket("multi-b")
+	mustCreateBucket(t, c, bucketA)
+	mustCreateBucket(t, c, bucketB)
+
+	putExpirationLifecycle(t, c, bucketA, "exp/", 1)
+	putExpirationLifecycle(t, c, bucketB, "exp/", 1)
+
+	const keyA = "exp/a.txt"
+	const keyB = "exp/b.txt"
+	putObject(t, c, bucketA, keyA, "a")
+	putObject(t, c, bucketB, keyB, "b")
+	backdateMtime(t, fc, bucketA, keyA, 30)
+	backdateMtime(t, fc, bucketB, keyB, 30)
+
+	out := runLifecycleShard(t)
+	t.Logf("shell output:\n%s", out)
+
+	// Both buckets must have their objects expired in this single sweep.
+	for _, c2 := range []struct {
+		bucket, key string
+	}{
+		{bucketA, keyA},
+		{bucketB, keyB},
+	} {
+		c2 := c2
+		require.Eventuallyf(t, func() bool {
+			_, err := c.HeadObject(context.Background(), &s3.HeadObjectInput{
+				Bucket: aws.String(c2.bucket), Key: aws.String(c2.key),
+			})
+			return err != nil
+		}, 30*time.Second, 500*time.Millisecond,
+			"%s/%s must be expired by the multi-bucket sweep", c2.bucket, c2.key)
+	}
+}