[terra-slide-panel] Find closest focusable element on click

[artpark]

Summary

What was changed:

• Updated set last clicked method to find the closest focusable element
• Updated Slide Panel Multiple Buttons Toggle test to use terra-buttons
• Updated wdio spec to remove body focus check

Why it was changed:
When disclosing a slide panel, the element that was clicked on is stored as the "last clicked" item. On terra-buttons, a is clicked instead of the , so the last clicked item is stored as a span. This change finds the closest focusable element by repeatedly checking its parent element.

Testing

This change was tested using:

• WDIO
• Jest
• Visual testing (please attach a screenshot or recording)
• Other (please describe below)
• No tests are needed

Used the Slide Panel Multiple Buttons Toggle test to verify changes

Reviews

In addition to engineering reviews, this PR needs:

• UX review
• Accessibility review
• Functional review

Additional Details

This PR resolves:

UXPLATFORM-10167

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