Issue 4520: Code block format loses syntax highlight

Goal

Verify whether issue #4520 is still reproducible on current main, determine whether PR #4837 actually fixes it, and if not, land the smallest test-first fix.

Tracker Context

• Source: GitHub issue #4520
• Title: Code block formatting causing syntax highlight to be lost
• Related PR: #4837 (fix(code-block): fix normalizer and insertBreak for syntax highlighting)
• Task type: bug investigation + fix
• Browser surface: yes, but start with code-level repro/tests

Relevant Instructions

• Use TDD for sane behavior seams.
• Load local learnings before non-trivial bug work.
• Closeout requires fresh same-turn verification.

Findings

• Issue symptom: formatting a JSON code block drops syntax highlighting.
• Existing open PR #4837 targets different symptoms:
  • normalizer wrapping bare text children into code_line
  • insertBreak resetting decorations
• Existing learning 2026-03-26-code-block-language-change-must-trigger-redecorate.md says cache reset alone is insufficient; editor.api.redecorate() is required after decoration-invalidating changes.
• Root cause confirmed in local repro: formatCodeBlock rewrote formatted JSON into a single code_line text node containing embedded \n, so decorate only had one structural line to target.
• Code-block formatter path was the real seam: packages/code-block/src/lib/formatter/formatter.ts

Plan

1. Inspect formatter, decoration cache, and current tests.
2. Reproduce missing highlight behavior in code-level tests.
3. Add failing regression test first.
4. Implement minimal fix.
5. Run targeted tests and required package verification.

Progress

• Fetched issue and PR context.
• Searched local learnings and found directly relevant redecorate guidance.
• Added a failing regression test for formatting JSON into multiple code lines.
• Updated formatCodeBlock to rebuild code_line children and call redecorate.
• Extracted the code-line replacement + redecorate path into a shared setCodeBlockContent transform and removed the stale local redecorate cast.
• Added a changeset and a solution note.

Verification Target

• Targeted bun test for touched code-block specs.
• Build-first package verification for touched packages if TypeScript changes.
• pnpm lint:fix.

Verification Results

• bun test packages/code-block/src/lib
• pnpm install
• pnpm turbo build --filter=./packages/code-block
• pnpm turbo typecheck --filter=./packages/code-block
• pnpm lint:fix
• pnpm check