fix: label unmarshaling for `BasicMergeRequest` list operations

mentioned in commit 8fb42366

mentioned in merge request gitlab-org/terraform-provider-gitlab!2697 (merged)

mentioned in merge request gitlab-org/cli!2422

mentioned in merge request gitlab-com/gl-infra/woodhouse!681

mentioned in merge request gitlab-org/release-tools!4498

mentioned in merge request gitlab-com/gl-infra/oncall-robot-assistant!104

mentioned in merge request fforster/epic-update!21

mentioned in merge request fforster/gitlab-mcp!174

mentioned in merge request schober-server/kubernetes/images/gitlab-token-refresher!35

mentioned in merge request gitlab-com/gl-infra/autolabels!102

mentioned in merge request gitlab-com/gl-infra/ringctl!357

mentioned in merge request gitlab-com/gl-infra/platform/runway/runwayctl!1245

mentioned in merge request sickit/token-operator!17