feat(ApplicationService): Add support for private folder hierarchies and discover()

This PR fixes issue #1317 where ApplicationService failed to access
applications inside private folders.

Key changes:
- Refactored _resolve_path() to properly handle mixed public/private
  folder hierarchies by probing path segments to find private boundaries
- Added _build_path_url() to construct correct URLs with PrivateContents
- Added _find_private_boundary() to detect where paths transition to private
- Added discover() method for exploring the Applications folder tree
- All application methods (get, get_document, exists, create, update,
  delete, rename) now correctly handle documents in private folders

The TM1 REST API requires different URL patterns:
- Public: /Contents('folder')/Contents('subfolder')
- Private: /PrivateContents('folder')/PrivateContents('subfolder')
- Mixed: /Contents('public')/PrivateContents('private')

Once inside a private folder, all nested content must use PrivateContents.

Author: nicolasbisurgi