Components.AI: Add per-message identity chrome and conversation semantics

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

Author name is never rendered, role styling is applied per turn, and the conversation lacks appropriate feed/log semantics.

### Describe the solution you'd like

Render optional avatar and author information per message, apply role-correct styling per message, and provide a documented, labeled conversation semantic structure compatible with virtualization and assistive technology.

### Additional context

Leaf task for #68227 requirements B4 and I2.

Start by reading #68227 requirements B4 and I2 and locating the Components.AI message component entry points. Review the existing rendering and semantics, then confirm completion when messages expose optional identity and role styling with labeled conversation semantics that remain usable with virtualization and assistive technology.

Written by the indexing model from the issue text.