feat(filer): add lazy directory listing for remote mounts (#8615)

* feat(filer): add lazy directory listing for remote mounts

Directory listings on remote mounts previously only queried the local
filer store. With lazy mounts the listing was empty; with eager mounts
it went stale over time.

Add on-demand directory listing that fetches from remote and caches
results with a 5-minute TTL:

- Add `ListDirectory` to `RemoteStorageClient` interface (delimiter-based,
  single-level listing, separate from recursive `Traverse`)
- Implement in S3, GCS, and Azure backends using each platform's
  hierarchical listing API
- Add `maybeLazyListFromRemote` to filer: before each directory listing,
  check if the directory is under a remote mount with an expired cache,
  fetch from remote, persist entries to the local store, then let existing
  listing logic run on the populated store
- Use singleflight to deduplicate concurrent requests for the same directory
- Skip local-only entries (no RemoteEntry) to avoid overwriting unsynced uploads
- Errors are logged and swallowed (availability over consistency)

* refactor: extract xattr key to constant xattrRemoteListingSyncedAt

* feat: make listing cache TTL configurable per mount via listing_cache_ttl_seconds

Add listing_cache_ttl_seconds field to RemoteStorageLocation protobuf.
When 0 (default), lazy directory listing is disabled for that mount.
When >0, enables on-demand directory listing with the specified TTL.

Expose as -listingCacheTTL flag on remote.mount command.

* refactor: address review feedback for lazy directory listing

- Add context.Context to ListDirectory interface and all implementations
- Capture startTime before remote call for accurate TTL tracking
- Simplify S3 ListDirectory using ListObjectsV2PagesWithContext
- Make maybeLazyListFromRemote return void (errors always swallowed)
- Remove redundant trailing-slash path manipulation in caller
- Update tests to match new signatures

* When an existing entry has Remote != nil, we should merge remote metadata   into it rather than replacing it.

* fix(gcs): wrap ListDirectory iterator error with context

The raw iterator error was returned without bucket/path context,
making it harder to debug. Wrap it consistently with the S3 pattern.

* fix(s3): guard against nil pointer dereference in Traverse and ListDirectory

Some S3-compatible backends may return nil for LastModified, Size, or
ETag fields. Check for nil before dereferencing to prevent panics.

* fix(filer): remove blanket 2-minute timeout from lazy listing context

Individual SDK operations (S3, GCS, Azure) already have per-request
timeouts and retry policies. The blanket timeout could cut off large
directory listings mid-operation even though individual pages were
succeeding.

* fix(filer): preserve trace context in lazy listing with WithoutCancel

Use context.WithoutCancel(ctx) instead of context.Background() so
trace/span values from the incoming request are retained for
distributed tracing, while still decoupling cancellation.

* fix(filer): use Store.FindEntry for internal lookups, add Uid/Gid to files, fix updateDirectoryListingSyncedAt

- Use f.Store.FindEntry instead of f.FindEntry for staleness check and
  child lookups to avoid unnecessary lazy-fetch overhead
- Set OS_UID/OS_GID on new file entries for consistency with directories
- In updateDirectoryListingSyncedAt, use Store.UpdateEntry for existing
  directories instead of CreateEntry to avoid deleteChunksIfNotNew and
  NotifyUpdateEvent side effects

* fix(filer): distinguish not-found from store errors in lazy listing

Previously, any error from Store.FindEntry was treated as "not found,"
which could cause entry recreation/overwrite on transient DB failures.
Now check for filer_pb.ErrNotFound explicitly and skip entries or
bail out on real store errors.

* refactor(filer): use errors.Is for ErrNotFound comparisons

weed/remote_storage/gcs/gcs_storage_client.go

@@ -131,6 +131,52 @@ func (gcs *gcsRemoteStorageClient) Traverse(loc *remote_pb.RemoteStorageLocation
 
 const defaultGCSOpTimeout = 30 * time.Second
 
+func (gcs *gcsRemoteStorageClient) ListDirectory(ctx context.Context, loc *remote_pb.RemoteStorageLocation, visitFn remote_storage.VisitFunc) (err error) {
+	pathKey := loc.Path[1:]
+	if pathKey != "" && !strings.HasSuffix(pathKey, "/") {
+		pathKey += "/"
+	}
+
+	objectIterator := gcs.client.Bucket(loc.Bucket).Objects(ctx, &storage.Query{
+		Delimiter: "/",
+		Prefix:    pathKey,
+		Versions:  false,
+	})
+
+	for {
+		objectAttr, iterErr := objectIterator.Next()
+		if iterErr != nil {
+			if iterErr == iterator.Done {
+				return nil
+			}
+			return fmt.Errorf("list directory %s%s: %w", loc.Bucket, loc.Path, iterErr)
+		}
+
+		if objectAttr.Prefix != "" {
+			// Common prefix → subdirectory
+			dirKey := "/" + strings.TrimSuffix(objectAttr.Prefix, "/")
+			dir, name := util.FullPath(dirKey).DirAndName()
+			if err = visitFn(dir, name, true, nil); err != nil {
+				return err
+			}
+		} else {
+			key := "/" + objectAttr.Name
+			if strings.HasSuffix(key, "/") {
+				continue // skip directory markers
+			}
+			dir, name := util.FullPath(key).DirAndName()
+			if err = visitFn(dir, name, false, &filer_pb.RemoteEntry{
+				RemoteMtime: objectAttr.Updated.Unix(),
+				RemoteSize:  objectAttr.Size,
+				RemoteETag:  objectAttr.Etag,
+				StorageName: gcs.conf.Name,
+			}); err != nil {
+				return err
+			}
+		}
+	}
+}
+
 func (gcs *gcsRemoteStorageClient) StatFile(loc *remote_pb.RemoteStorageLocation) (remoteEntry *filer_pb.RemoteEntry, err error) {
 	key := loc.Path[1:]
 	ctx, cancel := context.WithTimeout(context.Background(), defaultGCSOpTimeout)