fix: add DEFAULT_SLOT to TreeItem CheckboxContext so unslotted Checkboxes render

[mvanhorn]

Closes #10204

✅ Pull Request Checklist:

• Included link to corresponding React Spectrum GitHub Issue.
• Added/updated unit tests and storybook for this change (for new code or code which already has tests).
• Filled out test instructions.
• Updated documentation (if it already exists for this component). No documentation changes are needed for this internal slot fix.
• Looked at the Accessibility Practices for this feature - Aria Practices

📝 Test Instructions:

TreeItem provided a CheckboxContext/CheckboxFieldContext whose slots only contained selection, unlike TableRow and GridListItem which also include [DEFAULT_SLOT]: {}. As a result, any Checkbox or CheckboxField without slot="selection" rendered as a React descendant of a TreeItem threw A slot prop is required. Valid slot names are "selection".

This adds [DEFAULT_SLOT]: {} to both slot maps in TreeItem, mirroring Table.tsx, so an unslotted checkbox renders normally while the existing slot="selection" behavior is unchanged.

To verify:

1. Render a Tree with a TreeItem containing an unslotted <Checkbox> in its content. It now renders without throwing.
2. A Checkbox slot="selection" inside a TreeItem still drives selection (no regression).

Tests:

yarn jest packages/react-aria-components/test/Tree.test.tsx

Result: 109 passed, 1 skipped. Added a new case asserting an unslotted Checkbox under a TreeItem renders without error.

🧢 Your Project:

Community contribution.

[snowystinger]

Thank you!