fix(ui5-li): implement edit mode for keyboard navigation in list items

[kgogov]

Summary

Implements an edit mode for list items following the SAP Fiori Keyboard Interaction spec (Intentional Edit Pattern, p86-87):

• Navigation mode (default): Tab/Shift+Tab forwards out of the list. Arrow Up/Down navigates between items.
• Edit mode (toggled by F2 or F7): Tab flows through inner focusable elements within an item, then continues to the next item's elements, and exits the list after the last item. Shift+Tab flows backward symmetrically. Arrow Up/Down transfers focus to the same-index element in adjacent items.

The delete button in selectionMode="Delete" is now keyboard accessible — reachable via edit mode Tab flow without requiring the Delete key shortcut.

Changes

ListItem.ts

• Added _editMode private property to track edit/navigation mode state
• Overridden _handleTabNext — in edit mode, Tab flows to the next inner element or to the next item's first element via forward-after; in navigation mode, always fires forward-after
• Overridden _handleTabPrevious — in edit mode, Shift+Tab flows backward or to the previous item's last element via forward-before; in navigation mode, always fires forward-before
• Added _onfocusout handler to clear edit mode when focus leaves the component
• _handleF2 toggles edit mode on/off

List.ts

• onForwardAfter: when the source item is in edit mode, routes focus to the next item's first inner element instead of exiting the list
• onForwardBefore: when the source item is in edit mode, routes focus to the previous item's last inner element instead of exiting the list
• _handleF7: sets _editMode when entering/exiting internal elements
• _navigateToAdjacentItem: transfers _editMode state when arrow-navigating between items

ListItemBase.ts

• Made forward-before event cancelable (consistent with forward-after)

ListItemTemplate.tsx

• Removed tabindex={-1} and data-sap-no-tab-ref from the delete button, making it discoverable by getTabbableElements

List.cy.tsx

• Updated existing tests to reflect new Tab-forwarding behavior in navigation mode
• Added 10 new tests covering: F2/F7 toggle, Tab flow across items, Tab exit from last item, Shift+Tab backward flow, Arrow key edit mode transfer, focus-out cleanup, mixed item types, and a comprehensive workflow with ListItemCustom + custom delete button slot

ListEditMode.html

• New manual test page with 10 scenarios and a live focus tracker

Test plan

• All 99 List Cypress tests pass (0 failures, 0 regressions)
• NotificationList (fiori) — 31 tests pass
• Table (compat) — 36 tests pass
• Manual testing with ListEditMode.html test page
• Screen reader testing (VoiceOver / NVDA)

Fixes #13220

[ui5-webcomponents-bot]

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