Components.AI: Implement per-message and per-turn decorator slots

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

Applications cannot attach reusable content to each message or wrap an entire turn.

### Describe the solution you'd like

Implement the approved separate seams. Every request and assistant message gets a row beneath and outside its box aligned to that message; turn decorators wrap the whole turn and can render dividers, markers, or end-of-turn options.

### Additional context

Leaf task for #68227 requirements F1, F8, and F9. Depends on approved decorator API and stable identities.

Start with the requirements in #68227 and the approved decorator API, then inspect the existing Components.AI message and turn rendering entry points. Done means request and assistant messages expose aligned per-message slots, while turn decorators wrap the full turn and support dividers, markers, or end-of-turn options with stable identities.

Written by the indexing model from the issue text.