CmdPal: Restore correct focus behavior in Extensions settings page

### Description of the new feature / enhancement

Focus management in the Command Palette Extensions settings page should preserve user context when navigating with the keyboard.

When a user navigates through the extensions list and then leaves it, returning via Shift or Shift+Tab should restore focus to the previously selected extension card. Currently, focus may jump to the first or last item, which breaks navigation continuity.

This issue tracks re-implementation of a previously reverted change that attempted to address this behavior.

Previously implemented by #45903, but that has unintended side-effects.

### Scenario when this would be used?

Keyboard-only and assistive technology users can lose context due to unexpected focus movement. This increases navigation effort and reduces usability of the Extensions page.

### Supporting information

_No response_

Review the focus-management behavior described for the Command Palette Extensions settings page, then compare the previously implemented change in #45903 with the linked work in #46642. Confirm how keyboard navigation currently restores focus after leaving and returning to the extensions list; done means Shift and Shift+Tab restore focus to the previously selected extension card without the earlier side effects.

Written by the indexing model from the issue text.