Add unit tests for agent-service result-formatting

### Task Summary

`agent-service/src/agent/tools/result-formatting.ts` currently has ~0.93% unit-test coverage. Add unit tests for `formatOperatorResult` and the helpers it exercises (table formatting, input/output shape metadata, visualization-row stripping, sparse row gap markers, cell type coercion).

### Task Type

- [x] Testing / QA