WS-2486-Fix keyboard navigation order of new nav component

[Nabeel1276]

Resolves JIRA: https://bbc.atlassian.net/browse/WS-2486

Summary

Fixes keyboard navigation so what the focus does not go onto the 2nd tier but instead, the focus closes the menu and moves to the first item on the page as required.

Code changes

Added new closeMenuWhenFocusLeaves function for closing menu when focus leaves
Added unit test for the function

Testing

1. Navigate to a local BBC page that contains the new nav component - E.g http://localhost:7081/portuguese/articles/clyp0wdzgjlo?renderer_env=live.
2. Open dev tools and inspect on mobile
3. Tab through until focus leaves the nav
4. Nav should close upon focus leaving and focus should be on next item

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[Copilot]

Pull request overview

Fixes keyboard navigation order in the new canonical navigation component by closing the dropdown menu when keyboard focus moves outside of its container.

Changes:

• Adds an onBlur handler on the wrapper div around the top row and dropdown.
• Uses currentTarget.contains(relatedTarget) to detect focus leaving the menu subtree and closes the menu when it does.