Fix: required validation for multi-select ComboBox (#9788)

[Nawaz-B-04]

Closes #9788

✅ Pull Request Checklist:

• Included link to corresponding React Spectrum GitHub Issue.
• Added/updated unit tests and storybook for this change.
• Filled out test instructions.
• Updated documentation (not required for this fix).
• Looked at the Accessibility Practices for this feature.

📝 Test Instructions:

1. Create a form containing a ComboBox with selectionMode="multiple" and isRequired.
2. Select one or more items.
3. Submit the form.

Expected result:
Form submission succeeds when items are selected.

4. Submit the form with no selections.

Expected result:
Form validation prevents submission due to required constraint.

🧢 Your Project:

Personal open source contribution.