fix: accept readonly array ComboBox values

[lixiaoyan]

Summary

Re-applies the readonly array value type support from #9741 after the ComboBox state file was moved during the package consolidation work.

The consolidated react-stately/src/combobox/useComboBoxState.ts had kept most of the change, but ValueType<'multiple'> was restored to mutable Key[]. This updates it back to readonly Key[] so controlled multiple ComboBox values can use readonly arrays.

Closes

✅ 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

📝 Test Instructions:

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[snowystinger]

Thanks for catching that and creating a PR