Components.AI content rendering and tool-call presentation

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

Plain-text rendering is incomplete, supplied `RichTextNode` trees are ignored, and tool-call display behavior is accidental rather than an explicit extensibility contract.

### Describe the solution you'd like

Track default text correctness, rich-node renderer design and implementation, content safety, code blocks, markdown integration guidance, media-node integration, citations, and the three tool-call display modes defined by #68227.

This is a workstream under #68227. Its independently mergeable tasks are represented as native sub-issues.

### Additional context

Covers D1, D14-D15, F11, and J1-J8 from #68227.

Start with parent issue #68227 and review the native sub-issues that define the independently mergeable work. Map the relevant sub-issue to the listed rendering, safety, markdown, media, citation, or tool-call requirements; done means its specified scope is implemented and independently mergeable.

Written by the indexing model from the issue text.