Follow-up to #58161, which handed file drags to AppKit once the pointer
left the window but left the handoff one-way. Re-entering the source
window and dropping was not possible.

GPUI now suspends the original drag when handing it to the platform and
restores it when the drag re-enters the source window, so drop targets
receive the original typed payload instead of the synthetic
external-paths payload.

No release notes this hasn't shipped yet.

Release Notes:

- N/A