Remove redundant checkbox-input class from TreeCheckbox

The selection slot div in TreeCheckbox was using both className="checkbox-input"
and data-slot="selection", causing duplicate styling. The CSS already has
comprehensive [data-slot="selection"] rules that cover all the necessary
styling, making the checkbox-input class redundant and confusing.

Changes:
- Removed className="checkbox-input" from TreeCheckbox selection slot
- CSS [data-slot="selection"] rules already include all necessary styling
- Updated 5 test snapshots to reflect the cleaner markup

This addresses the Copilot review comment about duplicate/conflicting
styling between .checkbox-input class and [data-slot="selection"] attribute.

Author: OpenStaxClaude