Components.AI: Implement the extensible rich-node renderer

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

`RichContentBlock.Content` is ignored and only raw text is emitted.

### Describe the solution you'd like

Implement the approved renderer to walk supplied rich-content trees, render supported nodes, invoke per-kind extension points, preserve stable node identity, and update incrementally during streaming without replacing completed DOM.

### Additional context

Leaf task for #68227 requirements J2 and J6. Depends on approved renderer API and scoped streaming renders.

Start by reading the approved renderer API and the scoped streaming renders referenced by this issue, then review requirements J2 and J6 in #68227. Done means supported rich-content trees render through per-kind extension points, node identity remains stable, and streaming updates do not replace completed DOM.

Written by the indexing model from the issue text.