Standardize drag & drop behavior between Project Explorer and Finder (APP-4360)

[warp-dev-github-integration]

Description

Standardizes drag & drop behavior so that files dropped from the Project Explorer are handled identically to files dropped from Finder (the OS-level drag source).

Before:

• Project Explorer → Input: Inserted a relative path (relative to repo root) without shell escaping or image detection
• Project Explorer → Terminal (long-running command): Inserted the absolute path without shell escaping or image detection
• Finder → Input/Terminal: Properly detected images, applied shell escaping, handled WSL path conversion, and supported SSH file upload

After:
Both Project Explorer and Finder drops now go through the same code paths:

• ItemDroppedOnInput routes through the editor's DragAndDropFiles action, which handles image detection/attachment, shell escaping, WSL path transformation, and uses absolute paths
• ItemDroppedOnTerminal routes through TerminalView::drag_and_drop_files, which handles image detection, shell escaping, WSL path conversion, MSYS2 handling, and SSH file upload

Linked Issue

Resolves APP-4360

• The linked issue is labeled ready-to-spec or ready-to-implement.
• Where appropriate, screenshots or a short video of the implementation are included below (especially for user-visible or UI changes).

Testing

• Verified compilation with cargo check -p warp
• Verified formatting with cargo fmt
• The changes route through existing, well-tested code paths (EditorView::drag_and_drop_files and TerminalView::drag_and_drop_files) so no new test cases were needed

Agent Mode

• Warp Agent Mode - This PR was created via Warp's AI Agent Mode

Conversation: https://staging.warp.dev/conversation/05a569bf-0326-4e06-bef5-45d5e69eb591
Run: https://oz.staging.warp.dev/runs/019de638-7042-7076-a220-3c58e8b1bd2d
This PR was generated with Oz.

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