Replace Tab-through-every-item with a proper roving tabIndex +
arrow-key pattern per the WAI-ARIA toolbar pattern.

Keyboard handling

• [Left]/[Right] navigate between toolbar items (RTL-aware)
• [Home]/[End] jump to first/last item
• [Tab]/[Shift+Tab] exit the toolbar — browser native behaviour
  handles this naturally because only one item has tabIndex=0
• Overflow popover is fully isolated: arrow keys and focusin
  handling are suppressed when the popover is open, leaving the
  popover's natural Tab order intact

Roving tabIndex

• Only the current toolbar item has tabIndex=0; all others get -1
• _lastFocusedItem is restored on re-render so focus memory survives
  overflow layout changes
• When the overflow button disappears (items move back to toolbar),
  focus moves to the last navigable item via focusForToolbarNavigation
• Overflowed items are reset to tabIndex=0 so Tab works inside the
  overflow popover
• On popover open, first interactive overflow item receives focus

ToolbarItemBase — hook API for complex items
New overridable methods (all no-ops in base):

• getToolbarMovementInfo(): reports {currentIndex, itemCount} so the
  toolbar knows when a Left/Right press is at an internal boundary
• moveWithinToolbarItem(isForward): advances focus within the item
• focusForToolbarNavigation(isForward): direction-aware focus entry
• setToolbarForcedTabIndex(tabIndex): distributes roving tabIndex

ToolbarItem — three navigation paths

• _itemNavigation introspection: components that use ItemNavigation
  internally (e.g. SegmentedButton) are detected via duck-typing and
  their currentIndex/itemCount is read directly
• getToolbarMovementInfo interface: explicit opt-in for components
  that want to expose caret/selection position as boundary info
• Multi-child fallback: ToolbarItems with more than one slotted child
  (radio button groups, checkbox groups) are treated as a navigable
  group using _getCurrentNavigationState — no API needed on children

Input / TextArea — caret-aware boundary
Both implement getToolbarMovementInfo() using selectionStart and
value.length so Left/Right only exits to the next toolbar item once
the caret reaches the start or end of the text.

Templates
ToolbarButtonTemplate and ToolbarSelectTemplate: removed tabIndex JSX
prop — setting it on the UI5 host broke F6Navigation's focus traversal.

Docs

• Updated keyboard handling JSDoc on ui5-toolbar
• Added WCAG 2.1 notes to accessibleName / accessibleNameRef
• Restored @csspart root JSDoc entry on ui5-radio-button

Tests (Toolbar.cy.tsx)

• Arrow Left/Right navigation between items
• Home/End jump to first/last
• First overflow item focused on popover open
• Checkbox group: arrow traversal within group + boundary exit
• Overflow button: ArrowRight wraps to first item
• Updated "overflow button disappears" test to check focused
  ToolbarButton directly instead of shadow .ui5-tb-item wrapper

JIRA: BGSOFUIPIRIN-7018
Fixes: #12945