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/s3/s3_storage_client.go

@@ -1,10 +1,12 @@
 package s3
 
 import (
+	"context"
 	"fmt"
 	"io"
 	"net/http"
 	"reflect"
+	"strings"
 
 	"github.com/aws/aws-sdk-go/aws"
 	"github.com/aws/aws-sdk-go/aws/awserr"
@@ -98,12 +100,19 @@ func (s *s3RemoteStorageClient) Traverse(remote *remote_pb.RemoteStorageLocation
 				key := *content.Key
 				key = "/" + key
 				dir, name := util.FullPath(key).DirAndName()
-				if err := visitFn(dir, name, false, &filer_pb.RemoteEntry{
-					RemoteMtime: (*content.LastModified).Unix(),
-					RemoteSize:  *content.Size,
-					RemoteETag:  *content.ETag,
+				remoteEntry := &filer_pb.RemoteEntry{
 					StorageName: s.conf.Name,
-				}); err != nil {
+				}
+				if content.LastModified != nil {
+					remoteEntry.RemoteMtime = content.LastModified.Unix()
+				}
+				if content.Size != nil {
+					remoteEntry.RemoteSize = *content.Size
+				}
+				if content.ETag != nil {
+					remoteEntry.RemoteETag = *content.ETag
+				}
+				if err := visitFn(dir, name, false, remoteEntry); err != nil {
 					localErr = err
 					return false
 				}
@@ -122,6 +131,65 @@ func (s *s3RemoteStorageClient) Traverse(remote *remote_pb.RemoteStorageLocation
 	return
 }
 
+func (s *s3RemoteStorageClient) ListDirectory(ctx context.Context, loc *remote_pb.RemoteStorageLocation, visitFn remote_storage.VisitFunc) error {
+	pathKey := loc.Path[1:]
+	if pathKey != "" && !strings.HasSuffix(pathKey, "/") {
+		pathKey += "/"
+	}
+
+	listInput := &s3.ListObjectsV2Input{
+		Bucket:    aws.String(loc.Bucket),
+		Prefix:    aws.String(pathKey),
+		Delimiter: aws.String("/"),
+	}
+
+	var localErr error
+	listErr := s.conn.ListObjectsV2PagesWithContext(ctx, listInput, func(page *s3.ListObjectsV2Output, lastPage bool) bool {
+		for _, prefix := range page.CommonPrefixes {
+			if prefix.Prefix == nil {
+				continue
+			}
+			dirKey := "/" + strings.TrimSuffix(*prefix.Prefix, "/")
+			dir, name := util.FullPath(dirKey).DirAndName()
+			if err := visitFn(dir, name, true, nil); err != nil {
+				localErr = err
+				return false
+			}
+		}
+		for _, content := range page.Contents {
+			key := "/" + *content.Key
+			if strings.HasSuffix(key, "/") {
+				continue // skip directory markers
+			}
+			dir, name := util.FullPath(key).DirAndName()
+			remoteEntry := &filer_pb.RemoteEntry{
+				StorageName: s.conf.Name,
+			}
+			if content.LastModified != nil {
+				remoteEntry.RemoteMtime = content.LastModified.Unix()
+			}
+			if content.Size != nil {
+				remoteEntry.RemoteSize = *content.Size
+			}
+			if content.ETag != nil {
+				remoteEntry.RemoteETag = *content.ETag
+			}
+			if err := visitFn(dir, name, false, remoteEntry); err != nil {
+				localErr = err
+				return false
+			}
+		}
+		return true
+	})
+	if listErr != nil {
+		return fmt.Errorf("list directory %v: %w", loc, listErr)
+	}
+	if localErr != nil {
+		return fmt.Errorf("process directory %v: %w", loc, localErr)
+	}
+	return nil
+}
+
 func (s *s3RemoteStorageClient) StatFile(loc *remote_pb.RemoteStorageLocation) (remoteEntry *filer_pb.RemoteEntry, err error) {
 	resp, err := s.conn.HeadObject(&s3.HeadObjectInput{
 		Bucket: aws.String(loc.Bucket),