AI Deterministic Coverage Pass

Goal

Add a narrow, high-value non-React coverage pass for @platejs/ai focused on the deterministic lib seam:

• withAIBatch.ts
• getEditorPrompt.ts
• insertAINodes.ts
• replacePlaceholders.ts
• undoAI.ts
• removeAINodes.ts
• removeAIMarks.ts

Constraints

• Fast-lane only.
• No /react.
• No model mocks, network tests, or streaming coverage.
• Prefer tiny transform and helper contracts over wrapper coverage.

Slice

1. Add pure stub-based specs for withAIBatch and undoAI.
2. Add helper specs for prompt resolution and placeholder replacement.
3. Add real-editor specs for insert/remove AI node transforms.
4. Allow one small runtime fix if direct tests expose an actual bug.

Notes

• Existing non-React ai coverage is almost empty outside getMarkdown.spec.tsx.
• replacePlaceholders.ts currently uses String.prototype.replace, so repeated-placeholder coverage is likely to expose a real one-occurrence bug.
• Execution confirmed that repeated placeholders were in fact broken; the helper only replaced the first occurrence of {prompt} and the same bug pattern applied to other placeholder tokens.