feat(12.1-01): repair Nextcloud adapter contract and add URL normalization
- Remove incompatible NextcloudBackend.list_folder(folder_path="") override
so Nextcloud inherits canonical WebDAVBackend.list_folder(connection_id,
user_id, parent_ref=None, page_token=None) -> CloudListing
- Add normalize_nextcloud_url() to cloud_utils.py: idempotent canonical DAV
root derivation, username percent-encoding, https-only, rejects userinfo/
query/fragment, validates via SSRF guard before return
- Use normalize_nextcloud_url in cloud_backend_factory for nextcloud provider
- Add TestNextcloudBackendNoListFolderOverride and TestNextcloudUrlNormalization
to test_webdav_backend.py
- All 9 Nextcloud contract failures now pass; full focused suite: 158 passed
curo1305
2b46f74329