rust-status: fix a future compilation error

This was the warning by clippy 1.82.0, which explains the change:

```
warning: this function depends on never type fallback being `()`
   --> hg-core/src/dirstate_tree/status.rs:397:5
    |
397 | /     fn traverse_fs_directory_and_dirstate<'ancestor>(
398 | |         &self,
399 | |         has_ignored_ancestor: &'ancestor HasIgnoredAncestor<'ancestor>,
400 | |         dirstate_nodes: ChildNodesRef<'tree, 'on_disk>,
...   |
404 | |         is_at_repo_root: bool,
405 | |     ) -> Result<bool, DirstateV2ParseError> {
    | |___________________________________________^
    |
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
    = note: for more information, see issue #123748 <rust-lang/rust#123748>
    = help: specify the types explicitly
note: in edition 2024, the requirement `!: rayon::iter::FromParallelIterator<()>` will fail
   --> hg-core/src/dirstate_tree/status.rs:453:28
    |
453 |                 .collect::<Result<_, _>>()?;
    |                            ^^^^^^^^^^^^
    = note: `#[warn(dependency_on_unit_never_type_fallback)]` on by default
```

rust/hg-core/src/dirstate_tree/status.rs

@@ -450,7 +450,7 @@ impl<'a, 'tree, 'on_disk> StatusCommon<'a, 'tree, 'on_disk> {
                         }
                     }
                 })
-                .collect::<Result<_, _>>()?;
+                .collect::<Result<(), _>>()?;
 
             // We don’t know, so conservatively say this isn’t the case
             let children_all_have_dirstate_node_or_are_ignored = false;