refactor: fit popover in the viewport

[ugur-vaadin]

Description

This PR updates Popover's _updatePosition() to constrain overlays to viewport boundaries by calculating the centered position and adjusting if it would exceed left or right viewport edges.

Fixes #7750

Type of change

• Bugfix
• Feature
• Refactor

Checklist

• I have read the contribution guide: https://vaadin.com/docs/latest/contributing/pr
• I have added a description following the guideline.
• The issue is created in the corresponding repository and I have referenced it.
• I have added tests to ensure my change is effective and works as intended.
• New and existing tests are passing locally with my change.
• I have performed self-review and corrected misspellings.

[sonarqubecloud]

Quality Gate passed

Issues
8 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