Add unit test coverage for OperatorMenuService

[aglinxinyuan]

Task Summary

OperatorMenuService (frontend, workspace/service/operator-menu/) is ~30% line-covered. Extend its spec to cover the operator-menu actions over the workflow graph: disable/view-result/reuse-result toggle callbacks (both branches each), executeUpToOperator (no/one/many highlighted), saveHighlightedElements clipboard serialization (success + write rejection), and performPasteOperation (paste happy path incl. copy + position/overlap handling, empty clipboard, read rejection, and add-failure notification).

Task Type

• Refactor / Cleanup
• DevOps / Deployment / CI
• Testing / QA
• Documentation
• Performance
• Other