Windows: Fix canonicalize return UNC path

[CharlesChen0823]

In Windows platform, using notify to watch file events.

1. in notify windows implement, we get the full file path, just with path.join(file_path).
2. In zed worktree start_backgroud_scan_tasks, abs_path is not unc path, so we get all file events with not unc path.
3. but in zed worktree process_event, we strip_prefix unc path all times, it will always print annoy error.

@mikayla-maki I can't reopen pre closed pr #10501 .

Release Notes:

• N/A

[CharlesChen0823]

gpui::platform::windows::prompt_for_paths return not unc path, but i don't think here return unc path is better choice.

[mikayla-maki]

@CharlesChen0823 is this PR ready to go? The current fix looks a lot better and I'd be down to merge it :)

[CharlesChen0823]

@mikayla-maki yes, it's ready for review.

[mikayla-maki]

thank you!