test(s3/lifecycle): assert specific NotFound on multi-bucket deletion

Per codex review: TestLifecycleMultipleBucketsInOneSweep treated any
HeadObject error as "deleted", which lets a transport failure or
dead endpoint mask a real bug. Recognize NoSuchKey/NotFound/HTTP-404
specifically via a small isS3NotFound helper so the assertion
actually proves deletion happened, not just that the call broke.

test/s3/lifecycle/s3_lifecycle_multi_bucket_test.go

@@ -3,14 +3,40 @@ package lifecycle
 
 import (
 	"context"
+	"errors"
 	"testing"
 	"time"
 
 	"github.com/aws/aws-sdk-go-v2/aws"
 	"github.com/aws/aws-sdk-go-v2/service/s3"
+	"github.com/aws/aws-sdk-go-v2/service/s3/types"
+	smithyhttp "github.com/aws/smithy-go/transport/http"
 	"github.com/stretchr/testify/require"
 )
 
+// isS3NotFound recognizes a NotFound (NoSuchKey/404) response from the
+// AWS SDK. Treating any HeadObject error as "deleted" lets a transport
+// failure or dead endpoint mask a real bug, so callers that need to
+// prove deletion specifically should use this.
+func isS3NotFound(err error) bool {
+	if err == nil {
+		return false
+	}
+	var nsk *types.NoSuchKey
+	if errors.As(err, &nsk) {
+		return true
+	}
+	var notFound *types.NotFound
+	if errors.As(err, &notFound) {
+		return true
+	}
+	var apiErr *smithyhttp.ResponseError
+	if errors.As(err, &apiErr) {
+		return apiErr.HTTPStatusCode() == 404
+	}
+	return false
+}
+
 // 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
@@ -54,7 +80,10 @@ func TestLifecycleMultipleBucketsInOneSweep(t *testing.T) {
 			_, err := c.HeadObject(context.Background(), &s3.HeadObjectInput{
 				Bucket: aws.String(c2.bucket), Key: aws.String(c2.key),
 			})
-			return err != nil
+			// Pin: only count 404 / NoSuchKey as deletion. Any other
+			// error (transport failure, dead endpoint, auth) would
+			// otherwise mask a real bug as a "passed" test.
+			return isS3NotFound(err)
 		}, 30*time.Second, 500*time.Millisecond,
 			"%s/%s must be expired by the multi-bucket sweep", c2.bucket, c2.key)
 	}