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/serve_session.rs

@@ -349,9 +349,10 @@ impl ServeSession {
 
         let snap_start = Instant::now();
         log::trace!("Generating snapshot of instances from VFS");
+        let was_watch_enabled = vfs.is_watch_enabled();
         vfs.set_watch_enabled(false);
         let snapshot = snapshot_from_vfs(&instance_context, vfs, start_path)?;
-        vfs.set_watch_enabled(true);
+        vfs.set_watch_enabled(was_watch_enabled);
         log::debug!("Snapshot built in {:.1?}", snap_start.elapsed());
 
         vfs.clear_prefetch_cache();