message_editor: Fix image file copied from external files paste inserting filepath text alongside image (#51575)

What
---
Fix pasting image files copied from Finder in the Agent Panel on macOS.
Previously, pasting an image file copied with `Cmd+C` in Finder would
insert the file path as plain text instead of attaching the image as
context.

Why
---
Two bugs combined to cause this:

1. **Wrong clipboard type priority in `pasteboard.rs` (fixed in
#49367):** ~~The macOS pasteboard reader checked
`public.utf8-plain-text` first. When Finder copies a file it places the
filename as a string, the file path in `NSFilenamesPboardType`, and a
TIFF of the file icon — all in the same clipboard event. Because strings
were checked first, Zed read the filename string and ignored the image
data entirely.~~

2. **Missing `cx.stop_propagation()` in `message_editor.rs`:** The
`paste()` handler is registered as a `capture_action`. In GPUI's capture
phase, `propagate_event` defaults to `true` — simply `return`ing does
not stop propagation. Without an explicit `cx.stop_propagation()`, the
inner `Editor`'s bubble-phase paste handler also fired, read
`ExternalPaths` from the clipboard, converted it to text via
`ClipboardItem::text()` (which returns the file path string), and
inserted it alongside the image.

Fix
---
gpui_macos/src/pasteboard.rs`: Change clipboard read priority to **file
paths → image data → string**. Added `read_external_paths()` which reads
`NSFilenamesPboardType` and returns a `ClipboardEntry::ExternalPaths`.
This lets the existing `paste_images_as_context` path load the actual
file content.

- `agent_ui/src/message_editor.rs`: Add `cx.stop_propagation()` before
`task.detach()` when the image paste task is accepted, preventing the
inner editor from also handling the event.

Closes #51574

Test Plan
---

- [x] `cargo fmt --all -- --check`
- [x] `cargo build -p gpui_macos agent_ui` compiles clean
- [x] Manual verification of:
- [x] Copy an image file in Finder (`Cmd+C`), paste into Agent Panel —
image attaches correctly
- [x] Copy image from browser ("Copy Image"), paste into Agent Panel —
image attaches correctly
   - [x] `Cmd+Shift+4` screenshot paste still works
   - [x] Regular text paste still works
   
Release Notes
- Fixed pasting image files copied from Finder inserting the file path
instead of attaching the image in the Agent Panel

Screenshots
---

https://github.com/user-attachments/assets/edf6ba5a-6ff7-478c-a9ed-7cb5e889ccb3

<img width="801" height="388" alt="image"
src="https://github.com/user-attachments/assets/32d9321b-b0f6-47ae-a6b3-ea343cb69757"
/>

crates/agent_ui/src/message_editor.rs

@@ -869,7 +869,20 @@ impl MessageEditor {
             }
         }
 
+        let has_non_text_content = cx
+            .read_from_clipboard()
+            .map(|item| {
+                item.entries().iter().any(|entry| {
+                    matches!(
+                        entry,
+                        ClipboardEntry::Image(_) | ClipboardEntry::ExternalPaths(_)
+                    )
+                })
+            })
+            .unwrap_or(false);
+
         if self.prompt_capabilities.borrow().image
+            && has_non_text_content
             && let Some(task) = paste_images_as_context(
                 self.editor.clone(),
                 self.mention_set.clone(),
@@ -878,6 +891,7 @@ impl MessageEditor {
                 cx,
             )
         {
+            cx.stop_propagation();
             task.detach();
             return;
         }