Adds opt-in rendering for Markdown frontmatter metadata blocks in
Markdown Preview and agent markdown.

- Simple `key: value` metadata blocks now render as a two-column table,
while more complex metadata falls back to a code-style block.
- Metadata block content and key/value rows are parsed in the parser
step, and the request layout simply takes over rendering.

<img width="1288" height="436" alt="image"
src="https://github.com/user-attachments/assets/b35b949a-8bc4-47db-82ef-ed835e9ac06f"
/>

Release Notes:

- Added support for rendering Markdown frontmatter metadata blocks in
Markdown Preview and Agent Panel.