fix: wrong focus behavior when configuring menu selectability

[codewizardTM]

This PR fixes issue https://github.com/ant-design/ant-design/issues/53495

Determine the focus key based on the selectable configuration:

If the item is not selectable, focus the default item.

If the item is selectable, decide whether to focus based on whether there is already a selected item.

Summary by CodeRabbit

• Bug Fixes

  • 优化菜单组件焦点选择与聚焦行为：当支持可选项时优先聚焦已选项（若存在且有效），否则回退到活跃项或计算的默认焦点；当不支持可选项时始终使用计算的默认焦点，简化并统一聚焦优先级与回退逻辑。

• Tests

  • 新增针对可选/不可选场景的焦点交互测试，覆盖键盘导航与点击、聚焦顺序与 focus 调用验证。