fix: ensure menu-bar buttons always have correct tabindex

[web-padawan]

Description

Fixes #10355

Currently, vaadin-menu-bar checks for the first element with tabindex="0" in _setFocused() method. This doesn't work in cases when the button with this attribute gets hidden, especially an overflow button. Updated logic to always set tabindex attribute on the overflow button at the same time as hidden so they always stay in sync.

In addition to the problem reported in the above issue, I also added a test for another similar case originating from the same root issue: pressing Shift + Tab caused two tab stops rather than one (unexpected if tabNavigation isn't set).

menu-bar-shift-tab.mp4

Type of change

• Bugfix

[sonarqubecloud]

Quality Gate passed

Issues
6 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[vaadin-bot]

Hi @web-padawan and @web-padawan, when i performed cherry-pick to this commit to 24.8, i have encountered the following issue. Can you take a look and pick it manually?
Error Message:
Error: Command failed: git cherry-pick 686150f
error: could not apply 686150f... fix: ensure menu-bar buttons always have correct tabindex (#10356)
hint: After resolving the conflicts, mark them with
hint: "git add/rm ", then run
hint: "git cherry-pick --continue".
hint: You can instead skip this commit with "git cherry-pick --skip".
hint: To abort and get back to the state before "git cherry-pick",
hint: run "git cherry-pick --abort".