fix: stabilize flaky AI snapshot tests

Two root causes identified and fixed:

1. expectFileDeleted used file.exists() which checks the _stat cache.
   After unlink, _handleDirectoryChange re-reads the parent dir and
   can repopulate _stat on the deleted File object from a racing
   readdir. Switch to FileSystem.existsAsync() which bypasses the
   cached _stat and goes directly to the impl.

2. _createOrUpdateFile could fail with NotFound when file.exists()
   returned stale true after a delete+recreate cycle. Add fallback:
   if getDocumentForPath fails, create the file on disk and retry.

Author: abose

test/spec/ai-snapshot-test.js

@@ -133,12 +133,10 @@ define(function (require, exports, module) {
         }
 
         async function fileExists(name) {
-            return new Promise(function (resolve) {
-                const file = FileSystem.getFileForPath(toVfsPath(name));
-                file.exists(function (err, exists) {
-                    resolve(exists);
-                });
-            });
+            // Use FileSystem.existsAsync which bypasses the cached _stat on
+            // File objects — file.exists() can return stale true when
+            // _handleDirectoryChange re-populates _stat from a racing readdir.
+            return FileSystem.existsAsync(toVfsPath(name));
         }
 
         async function expectFileDeleted(name) {