fix(webdav): include fileid in find_by_type SEARCH + address PR #765 review

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>

tests/unit/client/test_nextcloud_client.py

@@ -172,10 +172,12 @@ class TestFindFilesByTag:
         for f in result:
             assert f["path"].startswith("/")
             assert f["last_modified_timestamp"] is not None
-        # SEARCH was scoped to the tagged folder (no leading slash).
+        # SEARCH was scoped to the tagged folder (no leading slash) and
+        # forwarded the requested MIME type as the positional first arg.
         client.webdav.find_by_type.assert_awaited_once()
-        call_kwargs = client.webdav.find_by_type.await_args.kwargs
-        assert call_kwargs["scope"] == "corpus"
+        call_args = client.webdav.find_by_type.await_args
+        assert call_args.args[0] == "application/pdf"
+        assert call_args.kwargs["scope"] == "corpus"
 
     async def test_dedupes_when_file_directly_tagged_and_under_tagged_folder(self):
         client = _make_client()