E2E tests: drag-and-drop notebook cell reordering

[rodrigosf672]

Summary

Extend the existing notebook-cell-reordering.test.ts suite with comprehensive drag-and-drop tests for issue #9840.

• 8 new drag-and-drop tests covering all E2E requirements from the issue
• Tests use test.step for clear organization with succinct assertions
• Adds dragCellToCell() helper method to page object with smart fallback selectors
• Consolidated with existing keyboard/action bar reordering tests in a single suite

Test Coverage

Existing tests (keyboard/action bar reordering):

• Action Bar: swap 1st and 2nd cell
• Keyboard: swap 1st and 2nd cell
• Boundaries: first-up and last-down are no-ops
• Multi-move: move first to end then one up
• Undo/redo cell move operation
• Multiselect: move multiple cells

New drag-and-drop tests:

• Drag-and-drop: single cell down by two positions
• Drag-and-drop: single cell up by one position
• Drag-and-drop: multiple selected cells
• Drag-and-drop: to top of notebook
• Drag-and-drop: to bottom of notebook
• Drag-and-drop: cancel with Escape key
• Drag-and-drop: auto-scroll when dragging near edges
• Drag-and-drop: undo/redo after drag operation

Test Plan

These tests are designed to run once the drag-and-drop feature is implemented. The dragCellToCell() helper includes fallback logic to adapt to the actual implementation:

• Primary: .cell-drag-handle (expected per issue spec)
• Fallback: .cell-gutter or .positron-notebook-code-cell-gutter
• Ultimate fallback: cell element itself

[github-actions]

E2E Tests 🚀
This PR will run tests tagged with: @:critical

^{readme  valid tags}

[rodrigosf672]

@rodrigosf672 draft for now, wait for clarifications and full implementation of #9840 to be settled so that changes to PR can be made for drag and drop tests to be automated (to catch potential breakages with that in the future).

[rodrigosf672]

#9840 not merged yet, wait for it to be ready for review and re-run those e2e tests (right now, they're all failing). Once fix is done, they should all pass. cc @nstrayer