Components.AI: Add accessible media loading, failure, and image affordances

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

Media lacks meaningful alt text, reserved dimensions, loading/failure states, and useful image viewing behavior, causing accessibility gaps and layout movement during streaming.

### Describe the solution you'd like

Reserve intrinsic dimensions, lazy-load where appropriate, render a descriptive failure fallback, expose real alt text, and provide an accessible expand/lightbox affordance using the shared media-rendering path.

### Additional context

Leaf task for #68227 requirements E4 and E6.

Start by locating the shared media-rendering path referenced by this issue and read requirements E4 and E6 from #68227. Confirm that media reserves intrinsic dimensions, lazy-loads where appropriate, exposes descriptive alt text, shows a failure fallback, and offers an accessible expand or lightbox affordance.

Written by the indexing model from the issue text.