When a single-file worktree's root file no longer exists, the background
scanner would previously enter an infinite retry loop attempting to
canonicalize the path. This caused continuous error logging and resource
waste.

This fix detects when a single-file worktree root cannot be
canonicalized (after attempting the file handle fallback) and emits a
new Deleted event, allowing the worktree to be properly closed.

This is most commonly encountered with temporary files, logs, and
similar files that are opened in Zed and then deleted externally, but
persist in the workspace database across sessions.

Closes #34864





## Test
**Logs** from manual testing:

```
2026-02-17T16:16:11+01:00 INFO  [worktree] inserting parent git repo for this worktree: "tmp.md"
2026-02-17T16:16:17+01:00 ERROR [worktree] root path could not be canonicalized: canonicalizing "/Users/***/tmp/tmp.md": No such file or directory (os error 2)
2026-02-17T16:16:17+01:00 INFO  [worktree] single-file worktree root "/Users/***/tmp/tmp.md" no longer exists, marking as deleted
2026-02-17T16:16:17+01:00 INFO  [worktree] worktree root /Users/***/tmp/tmp.md no longer exists, closing worktree
```
Release Notes:

- Fixed an issue where Zed would enter an infinite retry loop when the
backing file for a single-file worktree was deleted