Copying / Dragging image files (MacOS Terminal + iTerm) (#2567)

In this PR:

- [x] Add support for dragging / copying image files into chat.
- [x] Don't remove image placeholders when submitting.
- [x] Add tests.

Works for:

- Image Files
- Dragging MacOS Screenshots (Terminal, iTerm)

Todos:

- [ ] In some terminals (VSCode, WIndows Powershell, and remote
SSH-ing), copy-pasting a file streams the escaped filepath as individual
key events rather than a single Paste event. We'll need to have a
function (in a separate PR) for detecting these paste events.

codex-rs/Cargo.lock

@@ -1001,6 +1001,7 @@ dependencies = [
  "tui-markdown",
  "unicode-segmentation",
  "unicode-width 0.1.14",
+ "url",
  "uuid",
  "vt100",
 ]