test(12.1-01): add failing four-provider contract suite and fixtures
- Create test_cloud_provider_contract.py with parametrized suite for
Nextcloud, WebDAV, Google Drive, OneDrive
- Assert canonical (connection_id, user_id, parent_ref=None, page_token=None)
signature, CloudListing return type, trusted caller identity propagation,
metadata normalization, pagination completeness, and forbidden-operation spies
- Add synthetic credential-free fixtures: nextcloud_root.xml, webdav_root.xml,
google_drive_pages.json, onedrive_pages.json
- 9 tests fail for nextcloud (expected RED state — incompatible list_folder override)
- All webdav, google_drive, onedrive contract tests pass