Components.AI: Add extensible code-block rendering

### Is your feature request related to a problem? Please describe the problem.

Rich responses need usable code blocks without forcing markdown or a syntax-highlighting package into the core library.

### Describe the solution you'd like

Render code blocks with language metadata, horizontal scrolling, a copy action, and a syntax-highlighting extension point. Clipboard behavior must degrade explicitly when unavailable.

### Additional context

Leaf task for #68227 requirement J4. Depends on the rich-node renderer and clipboard interop.

Start with the rich-node renderer and clipboard interop referenced in the issue, then trace how Components.AI represents rich responses and code blocks. Done means code blocks expose language metadata, horizontal scrolling, copy behavior with an explicit unavailable-clipboard fallback, and an extension point for syntax highlighting.

Written by the indexing model from the issue text.