test: activate keyboard mode in popover focus visual tests (#11468) (CP: 25.1)

[vaadin-bot]

This PR cherry-picks changes from the original PR #11468 to branch 25.1.

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Original PR description

Summary

• Focus the popover target using sendKeys({ press: 'Tab' }) before opening, so that keyboard mode is active and the overlay :focus-visible ring is shown in screenshots
• After updating Playwright to 1.59 (Chrome 145+), the :focus-visible no longer applies because without keyboard activation, focus({ focusVisible: false }) is called. Since the popover overlay receives focus programmatically, the focus ring would disappear from screenshots without this change.

Test plan

• yarn test:base --group popover passes with updated screenshots

🤖 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

[vaadin-bot]

This ticket/PR has been released with Vaadin 25.1.2.