Components.AI: Correct turn/message semantics and assign stable identities

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

`ConversationTurnRenderer` classifies an entire turn as user or assistant, `ConversationTurn.Id` is not assigned, and rendered turns/messages/blocks are not keyed. This produces incorrect authorship styling and prevents reliable incremental rendering and virtualization.

### Describe the solution you'd like

- Model a turn as request message(s) plus all messages answering them.
- Keep authorship on each message, never on the turn.
- Assign stable identities to turns, messages, and blocks in the engine.
- Key each level in the render tree.
- Support multiple request messages in one turn for atomic queue drain.

### Additional context

Leaf task for #68227 requirements A2 and A7.

Start by tracing ConversationTurnRenderer, ConversationTurn, and the engine path that creates turns, messages, and blocks. Read requirements A2 and A7 in issue #68227, then follow how rendered turns, messages, and blocks are represented. Done means authorship is per message, identities are stable at every level, each render level is keyed, and multiple request messages can drain atomically.

Written by the indexing model from the issue text.