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/shell/command_remote_mount.go

@@ -48,6 +48,8 @@ func (c *commandRemoteMount) Help() string {
 	remote.mount -dir=/xxx -remote=cloud1/bucket -metadataStrategy=lazy
 	# mount and pull one directory in the bucket
 	remote.mount -dir=/xxx -remote=cloud1/bucket/dir1
+	# mount with on-demand directory listing cached for 5 minutes
+	remote.mount -dir=/xxx -remote=cloud1/bucket -listingCacheTTL=300
 
 	# after mount, start a separate process to write updates to remote storage
 	weed filer.remote.sync -filer=<filerHost>:<filerPort> -dir=/xxx
@@ -67,6 +69,7 @@ func (c *commandRemoteMount) Do(args []string, commandEnv *CommandEnv, writer io
 	nonEmpty := remoteMountCommand.Bool("nonempty", false, "allows the mounting over a non-empty directory")
 	metadataStrategy := remoteMountCommand.String("metadataStrategy", string(MetadataCacheEager), "lazy: skip upfront metadata pull; eager: full metadata pull (default)")
 	remote := remoteMountCommand.String("remote", "", "a directory in remote storage, ex. <storageName>/<bucket>/path/to/dir")
+	listingCacheTTL := remoteMountCommand.Int("listingCacheTTL", 0, "seconds to cache remote directory listings (0 = disabled)")
 
 	if err = remoteMountCommand.Parse(args); err != nil {
 		return nil
@@ -87,6 +90,7 @@ func (c *commandRemoteMount) Do(args []string, commandEnv *CommandEnv, writer io
 	if err != nil {
 		return err
 	}
+	remoteStorageLocation.ListingCacheTtlSeconds = int32(*listingCacheTTL)
 
 	strategy := MetadataCacheStrategy(strings.ToLower(*metadataStrategy))
 	if strategy != MetadataCacheLazy && strategy != MetadataCacheEager {