Fix Studio rendering of Codex control markers

[RyanNg1403]

Summary

• Hide Codex control metadata such as memory citations and turn-abort markers from Studio conversation text.
• Handle complete, unterminated, and split streamed markers without leaking or dropping surrounding user-visible text.
• Normalize legacy persisted and rollout messages, and remove marker-only bubbles.
• Add unit and Playwright regression coverage.

Validation

• npm test — 610 passed
• npm run lint
• npm run typecheck
• npx playwright test studio-ui sidebar studio-web --grep-invert=/pdf/i --workers=1 — 135 passed
• npm run studio:check
• git diff --check

npm run desktop:check is not available in this repository; no such script is defined.