Components.AI: Add built-in message and turn action menus

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

Applications must implement common message and turn actions and their composition behavior themselves.

### Describe the solution you'd like

Provide replaceable built-in message and turn menus assembled through decorators. Include the opt-in placement that appends turn-owned actions to the final assistant message row without replacing message-owned actions.

### Additional context

Leaf task for #68227 requirements F2 and F10. Depends on decorator slots.

Start by reading parent issue #68227, especially requirements F2 and F10, then inspect the decorator slots this task depends on. Define the replaceable message and turn menus, their composition behavior, and the opt-in placement for turn-owned actions; done means these requirements are covered without replacing message-owned actions.

Written by the indexing model from the issue text.