The default property set in `search_files` omits `<oc:fileid>`, so
`find_by_type` returned descendant dicts with no `file_id` — which
`NextcloudClient.find_files_by_tag` then silently dropped via its
dedup-by-id guard. Net effect: tag-on-folder produced zero expanded
descendants in CI (single-user / nc31, nc32). Mirrors the explicit
property list already used in `WebDAVClient.find_by_tag`.
Also addresses three nits from the PR #765 bot review:
- trim multi-paragraph docstring on `_normalise_search_result`
- trim multi-line docstring on `find_files_by_tag`
- match `is not None` ID-extraction pattern in the descendant loop
- assert positional `mime_type` arg in the unit test
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
