fix(a11y): avoid duplicate tab stops in toggle list tiles - #1100

The default switch, checkbox and radio controls create a second keyboard focus target inside their list tiles. Tabbing therefore stops twice on a single tile.

Wrap only the built-in controls in ExcludeFocus, leaving the tile responsible for keyboard focus and activation. Caller-supplied control widgets retain their own focus behavior.

Fixes #1083.

Validation with Flutter 3.47.5:

• Nine interaction tests cover forward/reverse traversal, Space activation, custom controls and disabled tiles across all three widget types.
• The three default traversal regressions fail against unchanged main and pass with this fix.
• Full suite: 655 tests passed, including existing goldens; no golden changes.
• flutter analyze --no-pub --fatal-infos, formatting and git diff --check passed.

Targeting main because this issue concerns the current widget API; the release branch still contains the older yaru_widgets package. Recent widget fixes also target main.

AI-assisted implementation and test execution.