Fix Code Block Language Rehighlight

Goal

Make code-block syntax highlighting refresh for the whole block immediately after lang changes.

Plan

• Add regression coverage around withCodeBlock language changes.
• Update withCodeBlock.apply to detect real lang transitions, clear cache, then trigger editor.api.redecorate().
• Verify with targeted tests, package build, package typecheck, and lint:fix.

Notes

• Package-level fix only.
• No registry UI changes.
• Type cleanup: redecorate now lives on the base SlateExtensionPlugin API as a no-op, so shared plugins can call editor.api.redecorate() without local casts.
• Verification:
  • 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 turbo typecheck --filter=./packages/core --filter=./packages/code-block failed on broader existing @platejs/code-block package type errors outside this cleanup
  • pnpm lint:fix