fix(contextual-menu): route Tab into the menu when opened with the mouse

[guillaume-flambard]

Done

Fixes #1363, following the smaller-step direction suggested in the issue thread by @edlerd: focus the first item when Tab is pressed while the menu is open and no menu item is focused yet.

• ContextualMenuDropdown renders through a portal, so after opening the menu with the mouse the menu items sit after every other page element in tab order: pressing Tab walked the rest of the page instead of entering the menu. Keyboard-opened menus already moved focus to the first item (they satisfy the detail === 0 gate), which is why the bug only reproduces after a mouse click.
• When the menu is open and Tab is pressed while focus is still on the toggle, focus now moves to the first menu item.
• Tab pressed anywhere else is untouched (guarded on activeElement being the toggle).
• Mouse behaviour is unchanged: no autofocus on click (the does not autofocus when opened by a mouse test still passes).
• The existing wrap-around logic is preserved and now lives in the same document-level listener, since the toggle sits outside the dropdown element.

The full ARIA menu pattern discussed in the issue (arrow-key navigation, roving tabindex) can build on this as a follow-up.

QA

Pinging @canonical/react-library-maintainers for a review.

Storybook

1. yarn start, open the ContextualMenu > Toggle story.
2. Click the toggle with the mouse (options must not take focus: unchanged).
3. Press Tab: focus now lands on the first menu option instead of walking the page.
4. Tab / Shift+Tab still wrap within the options; Escape still closes.

Tests

yarn jest src/components/ContextualMenu:

Test Suites: 2 passed, 2 total
Tests:       51 passed, 51 total

New tests: routes Tab into the menu when it was opened by a mouse (fails before the fix, passes after) and leaves Tab alone when focus is not on the toggle (guards against over-eager routing). eslint and tsc --noEmit clean.