Description

In Studio's agent chat, code blocks in assistant replies are unreadable in light mode. The code block header and body are hardcoded to white text (text-white) on a transparent background, so in light mode they render as white text on the white page (white-on-white).

Steps to reproduce

1. Open Studio and switch the theme to Light (Settings → Theme).
2. Chat with an agent so it replies with a fenced code block (e.g. a mermaid or python block).
3. The code is effectively invisible — white text on a white background.

Expected

Code blocks in the agent chat should stay readable in light mode and follow the active light/dark theme, like the rest of Studio.

Notes

• Affects the assistant-ui chat renderer: packages/playground/src/lib/ai-ui/messages/markdown-text.tsx.
• The same light-mode problem in the other code renderer (MarkdownRenderer, Shiki) is addressed separately in feat(playground-ui): CodeBlock as canonical code surface with CSS-driven light/dark highlighting #17430.