## Context

Dragging a file from the Project Panel into the agent composer was
appending the generated file mention to the end of the prompt instead of
inserting it at the active caret position. This change routes dragged
file insertion through the same caret-based mention insertion path
already used for pasted external paths, so dropped file and directory
mentions are inserted inline where the user is typing.

Closes #55002

Video of the manual test below :

[Screencast from 2026-04-29
01-13-59.webm](https://github.com/user-attachments/assets/9d2196fb-accd-4f4a-a9c0-0ce45b212121)

## How to Review

-
[crates/agent_ui/src/message_editor.rs](/home/saber/coding/zed/crates/agent_ui/src/message_editor.rs)
removes the separate end-of-buffer insertion mode from
`insert_mention_for_project_path`, updates `insert_dragged_files` to use
the shared caret-based insertion flow, and adds regression tests
covering single-file and multi-file drag insertion order at the cursor.
The existing paste-at-cursor behavior remains covered by the
pre-existing test in the same module.

## Self-Review Checklist

- [x] I've reviewed my own diff for quality, security, and reliability
- [ ] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the UI/UX checklist
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Release Notes:

- Fixed dragged file mentions in the agent composer being appended to
the end of the prompt instead of inserted at the cursor

Co-authored-by: Richard Feldman <richard@zed.dev>