fix(react-headless-components-preview): use global focusgroup typing for MenuListState

[Dmytro Kirpa (dmytrokirpa)]

Description

MenuListState locally intersected the base state with a hand-rolled focusgroup declaration:

export type MenuListState = MenuListBaseState & {
  root: {
    focusgroup?: string;
  };
};

This existed only to type the attribute assigned in useMenuList. But the repo-wide typings/focusgroup augmentation already adds focusgroup to React's HTMLAttributes, so the local override was:

• redundant — the property is already available on root
• weaker — string instead of the constrained FocusgroupAttribute union, so invalid values like focusgroup=\"tree\" wouldn't be caught
• noisy in the public API — it forced the base type to be re-exported under a MenuListState_2 alias

Now MenuListState is re-exported directly from @fluentui/react-menu, and useMenuList relies on the global typing.

Verification

• yarn nx run react-headless-components-preview:type-check passes
• etc/menu.api.md regenerated — the alias and the local intersection are gone
• Change file added

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