Add drag-and-drop reordering for schema form arrays

[devin-ai-integration]

This PR replaces the up/down button approach with drag-and-drop reordering for array items in the schema form using the @dnd-kit library. Array items now display a visible drag handle (GripVertical icon) that allows users to reorder items by dragging. The existing move up/down menu options are preserved as a fallback for accessibility.

Key implementation details:

• Prefix items (tuples) are excluded from drag-and-drop - they remain fixed in position with no drag handle
• Keyboard navigation is supported via KeyboardSensor with sortable keyboard coordinates
• Drag handle uses semantic color tokens (text-muted-foreground) for dark mode support
• Items show reduced opacity (0.5) while being dragged

Related to #15512

Test Coverage

Added comprehensive test coverage including:

• Snapshot tests for arrays with drag handles visible
• Snapshot tests verifying prefix items have no drag handles
• Behavioral tests for move up/down menu options
• Test verifying prefix items cannot be moved

Note: The tests cover the menu-based reordering and DOM structure. Manual verification is still recommended for the actual drag-and-drop interaction.

Visual Verification

Verified in Storybook that drag handles appear correctly for regular array items and are absent for prefix items:

Regular array items with drag handles:

Prefix items without drag handles:

Checklist

• This pull request references any related issue by including "closes <link to issue>"
• If this pull request adds new functionality, it includes unit tests that cover the changes
• If this pull request removes docs files, it includes redirect settings in mint.json.
• If this pull request adds functions or classes, it includes helpful docstrings.

Human Review Checklist

• Verify drag-and-drop works correctly in the browser for regular array items
• Verify prefix items (tuples) cannot be dragged and have no drag handle
• Verify keyboard navigation works for reordering (Tab to drag handle, then arrow keys)
• Verify move up/down menu options still function correctly
• Check drag handle appearance in both light and dark mode

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Requested by: @desertaxle
Link to Devin run: https://app.devin.ai/sessions/313e0a45d63a4af09c979a1f22eff8fd

[devin-ai-integration]

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