[HGrid] Custom summaries sample is missing custom summary

1. Open [Custom summaries, which access all data](https://staging.infragistics.com/products/ignite-ui-blazor/blazor/components/grids/hierarchical-grid/summaries#custom-summaries-which-access-all-data)
2. Observe mismatch between snipper and actual sample:

<img width="976" height="792" alt="Image" src="https://github.com/user-attachments/assets/39551418-b839-427f-b3c4-5e349cc76952" />
