Issue 4900: AI streaming history bloat

Source of truth

• Source type: GitHub issue
• Source id: #4900
• Title: Streaming with withAIBatch accumulates operations and may slow undo
• URL: https://github.com/udecode/plate/issues/4900
• Task type: bug / performance

Expected outcome

• Long AI streaming sessions should not build a giant undo batch that makes tf.ai.undo() or accept flows slower as chunk count grows.
• Keep current AI insert/chat behavior intact from a user point of view.

Constraints and repo rules

• Non-trivial task: use repo-local planning file, not root planning files.
• Check institutional learnings before implementation.
• Use a sane test seam before the fix if possible.
• If .ts changes, verify with install -> build -> typecheck sequence for touched package(s), then pnpm lint:fix.

Findings

• No relevant prior solution found in docs/solutions/.
• withAIBatch only merges batches and tags the last undo batch. It does not compact operations.
• Streaming insert mode is wrapped with withAIBatch(..., { split: isFirst }) in the AI kit integration.
• undoAI currently relies on the last undo batch being tagged as AI and calls native editor.undo().
• Accept/hide logic already distinguishes AI preview state from accepted content using AI marks and anchor cleanup.

Working hypothesis

• The real bug is treating streamed preview updates as normal history.
• Better fix: keep preview streaming out of history, then finalize or discard preview explicitly.
• If preview is unsaved, accept likely needs to remove preview without saving, then insert the final accepted content as one normal batch.

Next steps

1. Confirm accept/reject/reset paths needed for unsaved preview.
2. Add a failing test that proves streaming no longer bloats undo history.
3. Implement the smallest package-level fix.
4. Verify targeted tests, then package build -> typecheck -> lint.