Optional review hardening on #849 (non-blocking nits from the approve):
- `_build_search_xml`: emit `<d:firstresult>` on `offset is not None` rather
than truthiness, so a future explicit offset=0 isn't silently dropped.
- `_key`: key on `file_id is not None` so a (hypothetical) file_id of 0 isn't
treated as absent and mis-keyed onto path.
- `_type_search_args`: XML-escape the MIME type before interpolating it into
the SEARCH literal (defense-in-depth for any future user-supplied value),
with a unit test.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
