Code block language change must trigger redecorate

Problem

Changing code_block.lang updated the node value, but syntax highlighting for the block did not fully refresh right away.

Only after editing a line did the stale tokens disappear, so the bug looked like a per-line highlight problem instead of a decorate lifecycle gap.

Root cause

withCodeBlock.apply cleared CODE_LINE_TO_DECORATIONS when lang changed, but it stopped there.

That removed cached ranges, but Plate's React decorate function was still memoized behind versionDecorate. Without editor.api.redecorate(), Slate kept rendering with the old decoration pass until another change forced a fresh decorate cycle.

The original check also only looked at truthy operation.newProperties.lang, which missed some real transitions such as clearing or resetting the language.

Fix

Treat language changes as a two-step operation inside withCodeBlock.apply:

1. Detect a real lang transition by comparing previous and next values.
2. Clear cached line decorations before apply(operation).
3. Call editor.api.redecorate() after apply(operation) completes.

That makes these transitions all behave the same way:

• javascript -> json
• javascript -> plaintext
• undefined -> javascript
• javascript -> undefined

Verification

These checks passed:

bun test packages/core/src/lib/plugins/slate-extension/SlateExtensionPlugin.spec.tsx packages/code-block/src/lib/withCodeBlock.spec.tsx
pnpm install
pnpm turbo build --filter=./packages/core --filter=./packages/code-block
pnpm turbo typecheck --filter=./packages/core
pnpm lint:fix

This broader package check still fails on existing @platejs/code-block type errors outside this cleanup:

pnpm turbo typecheck --filter=./packages/core --filter=./packages/code-block

Prevention

When a plugin caches decoration state outside Slate nodes, clearing the cache is not enough. Also verify what actually triggers the next decorate pass.

For regressions around decorate, add one test that changes the underlying node data and asserts any explicit refresh hook is called. In this case, the stable seam is withCodeBlock.apply, not the UI combobox that calls setNodes({ lang }).

If a shared plugin needs a helper that only some editor runtimes override, put that helper on the base extension API as a harmless default first. Then feature packages can call the real contract directly instead of papering over the gap with local type assertions.