`NotebookItem::try_open` only checked the worktree-relative path's
extension, which is empty for single-file worktrees. As a result,
opening a notebook directly (`zed foo.ipynb`, or any open outside a
folder worktree) fell through to the text editor and showed raw JSON,
while the same file opened from within a folder workspace opened in the
notebook editor. Fall back to the absolute path when detecting
notebooks.

Tested locally (macOS, with the notebook feature enabled via
`LOCAL_NOTEBOOK_DEV`): `zed /tmp/solo.ipynb` now opens the rendered
notebook view; folder-workspace behavior unchanged.

Release Notes:

- Fixed notebooks opened as single files (e.g. `zed foo.ipynb`)
rendering as raw JSON instead of opening in the notebook editor
(preview-only).

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>