fix(aria/menu): prevent stale active state on menubar items

[Daherkarim]

Summary

The menubar's activeItem signal is set to the first item on init so the roving tabindex has an anchor, but data-active was bound directly to this anchor. That caused two visual bugs:

• The first menubar item rendered as active before any interaction.
• After hovering over an item and moving the pointer outside the menubar, the last hovered item stayed visually active because no mouseout handler reset the anchor.

This PR separates the tabstop concern from the visual highlight by tracking isHovered on MenuBarPattern (cleared on mouseout), and by gating MenuItemPattern.active for menubar parents on whether the menubar is focused, hovered, or the item is expanded. The roving tabindex behaviour is unaffected.

Reproduction

The bug reproduces on the official docs example: https://angular.dev/guide/aria/menubar#example-4

1. Load the page — the first menubar item is highlighted before any interaction.
2. Hover any other menubar item, then move the pointer outside the menubar — the last hovered item stays highlighted.

Test plan

• New unit tests in src/aria/menu/menu.spec.ts under Menu Bar Pattern > Active state covering:
  • no menubar item active on initial render
  • hovered item becomes active
  • active state clears on pointer leaving the menubar
  • moving between items keeps the new item active
  • focused menubar item is active
  • active state clears on focusout from the menubar
• pnpm bazel test //src/aria/menu:unit_tests passes on Chromium and Firefox.