fix: ListMultipartUploads pagination panic and duplicate results

This fixes a panic seen when there were a lot of multipart uploads in the
same bucket requiring multiple paginated responses. for example:
panic: runtime error: index out of range [11455] with length 1000
goroutine 418 [running]:
github.com/versity/versitygw/backend/posix.(*Posix).ListMultipartUploads(0xc0004300
/Users/ben/repo/versitygw/backend/posix/posix.go:2122 +0xd25
github.com/versity/versitygw/s3api/controllers.S3ApiController.ListActions({{0x183c
...

This change updates the ListMultipartUploads implementation to properly advance
past the (KeyMarker, UploadIDMarker) tuple when paginating, ensuring that each
response starts after the marker and does not include duplicate uploads.

backend/posix/posix.go

@@ -2117,28 +2117,33 @@ func (p *Posix) ListMultipartUploads(_ context.Context, mpu *s3.ListMultipartUpl
 		return uploads[i].Key < uploads[j].Key
 	})
 
-	for i := keyMarkerInd + 1; i < len(uploads); i++ {
-		if maxUploads == 0 {
-			break
+	start := 0
+	if keyMarker != "" {
+		for i, up := range uploads {
+			if up.Key == keyMarker && (uploadIDMarker == "" ||
+				up.UploadID == uploadIDMarker) {
+				// Start after the marker
+				start = i + 1
+				break
+			}
 		}
-		if keyMarker != "" && uploadIDMarker != "" && uploads[i].UploadID < uploadIDMarker {
-			continue
-		}
-		if i != len(uploads)-1 && len(resultUpds) == maxUploads {
+	}
+
+	for i := start; i < len(uploads); i++ {
+		if len(resultUpds) == maxUploads {
 			return s3response.ListMultipartUploadsResult{
 				Bucket:             bucket,
 				Delimiter:          delimiter,
 				KeyMarker:          keyMarker,
 				MaxUploads:         maxUploads,
-				NextKeyMarker:      resultUpds[i-1].Key,
-				NextUploadIDMarker: resultUpds[i-1].UploadID,
+				NextKeyMarker:      resultUpds[len(resultUpds)-1].Key,
+				NextUploadIDMarker: resultUpds[len(resultUpds)-1].UploadID,
 				IsTruncated:        true,
 				Prefix:             prefix,
 				UploadIDMarker:     uploadIDMarker,
 				Uploads:            resultUpds,
 			}, nil
 		}
-
 		resultUpds = append(resultUpds, uploads[i])
 	}