fix: preserve active attribute on menu-bar button while submenu is open (#11331) (CP: 25.0)

[vaadin-bot]

This PR cherry-picks changes from the original PR #11331 to branch 25.0.

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Original PR description

Summary

• Override _setActive() in MenuBarButton to skip deactivation while the button has the expanded attribute, preserving the pressed visual state while the sub-menu is open
• Fixes keyboard (Enter/Space) and mouse click interactions where ActiveMixin listeners were removing the active attribute immediately after _setExpanded() set it

Fixes #11315

Test plan

• Added tests verifying active attribute is preserved when opening submenu via Enter, Space, and click
• Added test verifying active attribute is removed when submenu closes
• All existing menu-bar tests pass (180/180)

🤖 Generated with Claude Code

[sonarqubecloud]

Quality Gate passed

Issues
0 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