- GoogleDriveBackend: list_folder with full pagination, native doc size=None, get_capabilities
- OneDriveBackend: list_folder with @odata.nextLink pagination, get_capabilities
- WebDAVBackend: list_folder via PROPFIND with SSRF re-validation, get_capabilities
- NextcloudBackend: inherits CloudResourceAdapter from WebDAVBackend
- build_cloud_resource_adapter() added to factory
- 22 new contract/behavior/pagination/no-byte-download tests (51 total pass)