fix(mount): keep a deferred local create from vanishing when its dir is rebuilt (#9991)

wfs.Create defers the filer create to Flush and inserts a local-only
placeholder into the metaCache (dirtyMetadata=true), so a just-created file
exists locally before the filer has it. When the parent directory falls out of
cache (hot-dir read-through, idle evict) and is rebuilt, EnsureVisited wipes the
store and refills from a filer listing that does not yet include the un-flushed
create, then markCachedFn publishes the directory authoritatively cached without
it. lookupEntry then returns an authoritative ENOENT and ReadDir returns nothing
— the file disappears from the mount although the client created it. Under
concurrent read+write churn on one directory this is the ConcurrentReadWrite
flake.

Preserve children the mount flags as local-only (open dirty handle or pending
async flush — the signal lookupEntry already trusts) across the rebuild's wipe
instead of blind-deleting them. Unpinned stale children are still dropped so a
rebuild cannot resurrect a deleted entry.

weed/mount/meta_cache/meta_cache_init.go

@@ -82,7 +82,7 @@ func doEnsureVisited(ctx context.Context, mc *MetaCache, client filer_pb.FilerCl
 				return
 			}
 			cleanupDone = true
-			if deleteErr := mc.DeleteFolderChildren(context.Background(), path); deleteErr != nil {
+			if deleteErr := mc.deleteFolderChildrenForRebuild(context.Background(), path); deleteErr != nil {
 				glog.V(2).Infof("clear %s build %s: %v", reason, path, deleteErr)
 			}
 			if abortErr := mc.AbortDirectoryBuild(context.Background(), path); abortErr != nil {
@@ -101,7 +101,7 @@ func doEnsureVisited(ctx context.Context, mc *MetaCache, client filer_pb.FilerCl
 
 		fetchErr := util.Retry("ReadDirAllEntries", func() error {
 			batch = nil // Reset batch on retry, allow GC of previous entries
-			if err := mc.DeleteFolderChildren(ctx, path); err != nil {
+			if err := mc.deleteFolderChildrenForRebuild(ctx, path); err != nil {
 				return fmt.Errorf("clear existing entries for %s: %w", path, err)
 			}
 			var err error