Suggestion menu omits aria-activedescendant for the first item

[pproenca]

Affected version

@blocknote/react@0.52.1 and current main at dee7880b89b1e9bc00b4f4481f32652c7a4b4408.

Problem

Both SuggestionMenuWrapper and GridSuggestionMenuWrapper conditionally set
aria-activedescendant using the truthiness of selectedIndex. The initial
selected index is numeric 0, so the editor contenteditable omits the
attribute even though the first suggestion is selected.

Affected sources:

• packages/react/src/components/SuggestionMenu/SuggestionMenuWrapper.tsx
• packages/react/src/components/SuggestionMenu/GridSuggestionMenu/GridSuggestionMenuWrapper.tsx

Reproduction

1. Mount BlockNoteViewRaw with the public SuggestionMenuController.
2. Open a standard slash suggestion menu with at least one item.
3. Inspect the editor contenteditable while the first item is selected.
4. Repeat with the grid suggestion-menu wrapper.

The first item is visibly selected, but aria-activedescendant is absent.
Moving to index 1 makes the attribute appear.

Expected

aria-activedescendant="bn-suggestion-menu-item-0"

No selection should continue to omit the attribute.

Cause and minimal fix

The wrappers currently use a truthy check:

"aria-activedescendant": selectedIndex
  ? "bn-suggestion-menu-item-" + selectedIndex
  : undefined,

Use an explicit undefined check:

"aria-activedescendant":
  selectedIndex !== undefined
    ? "bn-suggestion-menu-item-" + selectedIndex
    : undefined,

Tests should cover first, middle, last, and no-selection states for both the
standard and grid wrappers.

This affects assistive technology because the active-option relation is
missing until keyboard navigation reaches a nonzero index.