fix(s3): round-trip null-version delete-marker identity (#6765)

* fix(s3): round-trip null-version delete-marker identity through listing and delete responses

On a versioning-suspended bucket, a null delete marker's identity was lost on the way back to the client at three points (issue #6745): ListObjectVersions advertised the marker's VersionId as the literal nil UUID instead of null; deleting by that id succeeded but the DeleteObjects/DeleteObject response reported the identity as null with no way to correlate it to the request; and the response lacked DeleteMarker/DeleteMarkerVersionId because the marker-ness comparison mixed the client-facing identity (Some(nil)) with the storage identity (None), so the removal also mis-recorded accounting and fired DeleteMarkerCreated semantics on later paths.

- Listing (bucket_usecase, s3_api/bucket, build_list_versions_next_marker) now maps the synthesized nil UUID to the literal null everywhere it reaches the wire, and VersionMarker::parse folds a nil-UUID marker from older listings into VersionMarker::Null so pagination resumes correctly.
- delete_objects normalizes both sides of the marker-ness comparison via delete_file_info_version_id (matching the adjacent explicit_delete_marker admission check) and reports DeleteMarkerVersionId as null for an explicit null-marker removal.
- resolve_delete_version_state reports delete_marker for an explicit-version delete whose target is a delete marker even when the bucket is versioning-suspended, fixing x-amz-delete-marker on the single-object path.
- The DeleteObjects response entry echoes the version identity the request addressed for marker removals, marker-removal accounting no longer records a marker creation, and notification events fire DeleteMarkerCreated only for actual marker creation.

Fixes #6745

* fix(s3): keep null-marker removal write shape undeleted and report marker semantics response-side

The first cut marked the storage delete request deleted for a null-marker removal, which FileMeta::delete_version interprets as the suspended-bucket delete-mints-a-marker write and re-creates the marker just removed. Carry marker-ness to responses via explicit_delete_removed_marker (single path) and a response-only branch flag (batch path) instead, keeping every storage write shape byte-identical to the pre-fix behavior. Adds an embedded end-to-end regression test covering the full issue #6745 round trip.

crates/storage-api/src/object.rs

@@ -167,7 +167,15 @@ impl VersionMarker {
         if marker == NULL_VERSION_MARKER {
             Ok(Self::Null)
         } else {
-            Ok(Self::Version(Uuid::parse_str(marker)?))
+            let version = Uuid::parse_str(marker)?;
+            // Older releases advertised the null version as a nil UUID
+            // (issue #6745); a stored null version has no UUID, so resuming
+            // by `Version(nil)` could never match. Fold it into `Null`.
+            if version.is_nil() {
+                Ok(Self::Null)
+            } else {
+                Ok(Self::Version(version))
+            }
         }
     }
 }
@@ -714,6 +722,19 @@ fn is_modified_since(mod_time: &OffsetDateTime, given_time: &OffsetDateTime) ->
 mod tests {
     use super::*;
 
+    #[test]
+    fn version_marker_parse_folds_null_and_nil_uuid_into_null() {
+        assert_eq!(VersionMarker::parse("null"), Ok(VersionMarker::Null));
+        // Older releases advertised the null version as a nil UUID
+        // (issue #6745); it must resume as the null marker, not a UUID no
+        // stored version carries.
+        assert_eq!(VersionMarker::parse(Uuid::nil().to_string()), Ok(VersionMarker::Null));
+
+        let version = Uuid::from_u128(7);
+        assert_eq!(VersionMarker::parse(version.to_string()), Ok(VersionMarker::Version(version)));
+        assert!(VersionMarker::parse("not-a-version").is_err());
+    }
+
     #[test]
     fn http_preconditions_ignore_empty_etag_headers() {
         let opts = HTTPPreconditions {