Fix CheckboxGroup form reset with multiple selections

[Bhavani2016]

Closes #10643

When a form resets a CheckboxGroup with multiple selected values, one checkbox can remain selected. The goal is for every checkbox to return to the group's default selection.

removeValue previously calculated each removal from the values captured by the render. When multiple removals happen in the same update, a later removal can restore a value removed earlier. This change uses a functional state update so each removal sees the latest values.

The change adds a React Aria Components regression test for both CheckboxField and Checkbox, plus a React Stately test for multiple removals in one update.

AI assistance disclosure: I used ChatGPT to help investigate the issue

✅ 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).
• Looked at the Accessibility Practices for this feature - Aria Practices
• I understand every change in this PR and can explain why it's there.
• If AI-assisted, I followed our AI contribution guidance and pointed my assistant at AGENTS.md.

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