When shared_folders was enabled and the remote was mounted at root,
listing subdirectories would return top-level shared folder names
instead of the directory contents, causing "Entry doesn't belong in
directory (too short)" errors. Opening and reading files inside shared
folders also failed with path/not_found because the API calls didn't
use the shared folder's namespace.
Now when listing a subdirectory, use the shared folder's namespace ID
to create a per-request files client and list its contents directly
via the Dropbox files API. This works for all shared folder types
including team folders.
Fix NewObject to resolve files inside shared folders using the correct
namespace. Store the namespace ID on Object so that Open/Download also
uses the correct namespace-scoped client.
Extract the common directory listing logic into a listDir helper
method to avoid code duplication between the normal and shared folder
listing paths.
Add integration tests that share a folder, then verify listing,
NewObject, and Open all work in shared_folders mode.
14b56fa7dc