Add is_watch_enabled method to VFS and enhance prefetch cache tests

- Introduced `is_watch_enabled` method to check the status of automatic file watching in the VFS.
- Added a new test for prefetch cache to validate metadata retrieval and error handling for walked roots and external paths.
- Updated the `watch` method in `StdBackend` for cleaner logic.
- Enhanced the `dir_init` field in `PrefetchCache` to store resolved init-file information for directories, improving prefetch operations.

src/snapshot_middleware/mod.rs

@@ -544,6 +544,12 @@ mod test {
                 "DEFAULT_PROJECT_NAMES entry {project_name:?} missing from INIT_FILE_PRIORITY"
             );
         }
+        for &name in &priority_names {
+            assert!(
+                DEFAULT_PROJECT_NAMES.contains(&name),
+                "INIT_FILE_PRIORITY has Project entry {name:?} not in DEFAULT_PROJECT_NAMES"
+            );
+        }
     }
 
     #[test]