Components.AI: Add drag-and-drop attachment acquisition

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

Users can only choose attachments through the file picker.

### Describe the solution you'd like

Add a drag-and-drop target and drag-over affordance using native input and CSS behavior where possible, feeding the same validation and attachment-state pipeline as file selection.

### Additional context

Leaf task for #68227 requirement D4.

Start at the Components.AI attachment UI and trace the existing file-picker path through its validation and attachment-state pipeline. Use the native input and CSS behavior described in the issue as the reference; done means users can drag files onto the target, see a drag-over affordance, and receive the same validation and state updates as file selection.

Written by the indexing model from the issue text.