Support screenshot drag-and-drop as image attachments in Agent Panel

Reproduction steps

1. Open a Zed workspace on macOS.
2. Start an agent thread whose ACP prompt capabilities do not include image: true.
3. Take a screenshot with macOS.
4. Drag the floating screenshot thumbnail into the Agent Panel message input.

Current vs. Expected behavior

Actual behavior

Nothing is added to the input and no feedback is displayed.

Expected behavior

Display a clear, dismissible message such as:

The current agent does not support image attachments.

For agents that advertise image support, the screenshot should be added as an image attachment.

Zed version and system specs

Zed 1.20.1 Commit: f85546becf452a4732ffeee2302ccbfa5c38b397 Version: 1.20.1+stable.359.f85546becf452a4732ffeee2302ccbfa5c38b397