DnD missing-context runtime guard

Goal

Prevent @platejs/dnd from throwing Expected drag drop context when a Plate surface renders without an active react-dnd manager, while keeping normal browser DnD behavior intact.

Findings

• apps/www already wraps the app tree in Providers with DndProvider.
• The editor DnD plugin also wraps aboveSlate in dnd-kit.tsx with DndProvider.
• Prior repo learning in 2026-03-28-next-prerendered-client-editors-need-dnd-hooks-to-noop-on-the-server.md already established that this error is often a runtime-environment contract bug, not missing provider plumbing.
• Current package fix: guard useDragNode and useDropNode on both DOM availability and DndContext.dragDropManager.

Plan

1. Confirm the hook-level guard is the right durable fix.
2. Run remaining verification for touched surfaces.
3. State clearly whether the duplicate-deps hypothesis is supported by evidence.

Progress

• Added a missing-context guard in useDragNode.ts and useDropNode.ts.
• Switched the guard to import DndContext from react-dnd/dist/core/index.js because Bun did not reliably surface that export from the package root during tests.
• Extended useDraggable.spec.tsx with a missing-context regression case.
• Initial targeted Bun test hit the repo's known mixed .bun / .pnpm React invalid-hook-call failure mode; pnpm run reinstall cleared it.
• Final verification passed:
  • pnpm --filter @platejs/dnd test -- useDraggable.spec.tsx
  • bun test packages/dnd/src/components/useDraggable.spec.tsx
  • pnpm turbo build --filter=./packages/dnd --filter=./apps/www
  • pnpm turbo typecheck --filter=./packages/dnd --filter=./apps/www
  • pnpm lint:fix
  • browser-use --connect http://127.0.0.1:9222 verification on / and /cn with no console errors, no page errors, and no Expected drag drop context crash text