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/pb/remote_pb/remote.pb.go

@@ -457,12 +457,13 @@ func (x *RemoteStorageMapping) GetPrimaryBucketStorageName() string {
 }
 
 type RemoteStorageLocation struct {
-	state         protoimpl.MessageState `protogen:"open.v1"`
-	Name          string                 `protobuf:"bytes,1,opt,name=name,proto3" json:"name,omitempty"`
-	Bucket        string                 `protobuf:"bytes,2,opt,name=bucket,proto3" json:"bucket,omitempty"`
-	Path          string                 `protobuf:"bytes,3,opt,name=path,proto3" json:"path,omitempty"`
-	unknownFields protoimpl.UnknownFields
-	sizeCache     protoimpl.SizeCache
+	state                  protoimpl.MessageState `protogen:"open.v1"`
+	Name                   string                 `protobuf:"bytes,1,opt,name=name,proto3" json:"name,omitempty"`
+	Bucket                 string                 `protobuf:"bytes,2,opt,name=bucket,proto3" json:"bucket,omitempty"`
+	Path                   string                 `protobuf:"bytes,3,opt,name=path,proto3" json:"path,omitempty"`
+	ListingCacheTtlSeconds int32                  `protobuf:"varint,4,opt,name=listing_cache_ttl_seconds,json=listingCacheTtlSeconds,proto3" json:"listing_cache_ttl_seconds,omitempty"` // 0 = disabled; >0 enables on-demand directory listing with this TTL in seconds
+	unknownFields          protoimpl.UnknownFields
+	sizeCache              protoimpl.SizeCache
 }
 
 func (x *RemoteStorageLocation) Reset() {
@@ -516,6 +517,13 @@ func (x *RemoteStorageLocation) GetPath() string {
 	return ""
 }
 
+func (x *RemoteStorageLocation) GetListingCacheTtlSeconds() int32 {
+	if x != nil {
+		return x.ListingCacheTtlSeconds
+	}
+	return 0
+}
+
 var File_remote_proto protoreflect.FileDescriptor
 
 const file_remote_proto_rawDesc = "" +
@@ -573,11 +581,12 @@ const file_remote_proto_rawDesc = "" +
 	"\x1bprimary_bucket_storage_name\x18\x02 \x01(\tR\x18primaryBucketStorageName\x1a]\n" +
 	"\rMappingsEntry\x12\x10\n" +
 	"\x03key\x18\x01 \x01(\tR\x03key\x126\n" +
-	"\x05value\x18\x02 \x01(\v2 .remote_pb.RemoteStorageLocationR\x05value:\x028\x01\"W\n" +
+	"\x05value\x18\x02 \x01(\v2 .remote_pb.RemoteStorageLocationR\x05value:\x028\x01\"\x92\x01\n" +
 	"\x15RemoteStorageLocation\x12\x12\n" +
 	"\x04name\x18\x01 \x01(\tR\x04name\x12\x16\n" +
 	"\x06bucket\x18\x02 \x01(\tR\x06bucket\x12\x12\n" +
-	"\x04path\x18\x03 \x01(\tR\x04pathBP\n" +
+	"\x04path\x18\x03 \x01(\tR\x04path\x129\n" +
+	"\x19listing_cache_ttl_seconds\x18\x04 \x01(\x05R\x16listingCacheTtlSecondsBP\n" +
 	"\x10seaweedfs.clientB\n" +
 	"FilerProtoZ0github.com/seaweedfs/seaweedfs/weed/pb/remote_pbb\x06proto3"