* fix(s3api/list): cancel ListEntries stream in hasChildren
* fix(s3api): use filer_pb.List in hasChildren
filer_pb.List already wraps the ListEntries stream in a cancellable
context, so the single-entry probe needs no separate helper or manual
context plumbing to avoid the leaked gRPC stream goroutine.
* fix(s3api): propagate request context into hasChildren
Thread r.Context() through listFilerEntries and hasChildren so the
implicit-directory probe cancels when the client disconnects, instead
of running on context.Background().
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Co-authored-by: Chris Lu <chris.lu@gmail.com>
