Components.AI: Add composer overflow and responsive input-part layout

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

App-contributed composer controls can crowd out essential controls at narrow widths, and there is no reusable overflow/menu primitive.

### Describe the solution you'd like

Provide a composable menu/overflow input part and responsive contribution policy. At phone widths, essential built-ins retain their positions while eligible app-contributed controls move behind overflow.

### Additional context

Leaf task for #68227 requirement C2 and the narrow composer behavior in D10.

Start by reading requirement C2 in #68227 and the narrow composer behavior described in D10. Then locate the Components.AI composer entry points and any related tests, which the issue does not name. Done means a reusable overflow input part and responsive contribution policy preserve essential built-ins at phone widths while eligible app-contributed controls move behind overflow.

Written by the indexing model from the issue text.