feat(table): add support for group rows

[aborjinik]

Introduces the ui5-table-group-row component, which allows visually grouping rows under a shared header within the table.

What it does

ui5-table-group-row is a new component that extends TableRow and renders a single cell spanning all visible columns. It acts as a visual separator and group label between sets of data rows.

Key design decisions

• Extends TableRow — Reuses the existing row rendering pipeline (grid layout, focus handling, aria-rowindex) without special-casing in Table core since all the features provided by the row (selection, row actions, interactive etc.) will be needed by the TableGroupRow soon.
• Opt-out pattern — Group rows override getters (_isSelectable,_hasPopin, _isInteractive, _isNavigable, _hasSelector) to disable features that don't apply, keeping feature code unchanged.
• Empty cells/actions — Object.defineProperty overrides the cells an actions slots with frozen empty arrays, preventing accidental slot content from rendering.
• isGroupRow() marker — Used by Table core and features to detect group rows without instanceof checks across shadow DOM.

Integration with existing features

• Selection — Group rows are excluded from Select All and range selection. TableSelectionMulti filters them via isGroupRow().
• Row alternation — The alternating color counter resets after each group row so groups have independent zebra striping.
• Keyboard navigation — Group rows participate in the grid navigation as normal rows (focusable via arrow keys).
• Custom announcements — Screen reader announcements include group rows in the row count but label them via aria-roledescription.
• Popin mode — Group rows report _hasPopin = false and are skipped during popin state sync to avoid errors.
• Drag and Drop — Group rows are draggable alongside regular rows.

Accessibility

• role="row" with aria-roledescription set to a translatable "Group Row" string.
• Single gridcell with aria-colspan spanning all visible columns and aria-colindex="1".
• Participates in sequential aria-rowindex numbering.

Files changed

• New: TableGroupRow.ts, TableGroupRowTemplate.tsx, TableGroupRow.css
• Tests: TableGroupRow.cy.tsx
• Docs/Samples: Grouping sample, TableGroupRow API docs

Fixes: #10148
CPOUIFTEAMB-2446

[ui5-webcomponents-bot]

🚀 Deployed on https://pr-13510--ui5-webcomponents-preview.netlify.app

[KvM2]

Just a few small comments including some questions.