fix(storage): the flush watcher cannot arm twice in its async window

Arming is asynchronous — the request directory is created before it can be
watched — so during that window neither the watcher nor the sweep interval
exists yet and the guard let a second call through, leaving two watchers and
two sweeps for the life of the store. The callback is the flag that covers the
window.

src/storage/adapters/fileSystemStorage.ts

@@ -2432,7 +2432,12 @@ export class FileSystemStorage extends BaseStorage {
    *     an inspector whose request is never seen waits forever.
    */
   public override startFlushRequestWatcher(onRequest: () => Promise<void>): void {
-    if (this.flushWatcherInterval || this.flushWatcher) return // already watching
+    // Already watching — or already ARMING. The arm is asynchronous (the
+    // request directory is created before it can be watched), so neither the
+    // watcher nor the interval exists yet during that window; the callback is
+    // the flag that covers it. Without this a second call in the window would
+    // leave two watchers and two sweeps running for the life of the store.
+    if (this.flushWatcherInterval || this.flushWatcher || this.flushWatcherOnRequest) return
     this.flushWatcherOnRequest = onRequest
 
     const reqDir = path.join(this.lockDir, FileSystemStorage.FLUSH_REQUEST_DIR)