fix(webdav): harden offset/key truthiness and escape SEARCH mime type

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>

tests/unit/test_webdav_search_paging.py

@@ -164,6 +164,17 @@ async def test_find_all_by_type_delegates_to_search_files_all(mocker):
     assert "application/pdf" in kwargs["where_conditions"]
 
 
+def test_type_search_args_escapes_mime_type(mocker):
+    """A MIME value with XML metacharacters must not break / inject into the SEARCH."""
+    client = _make_client(mocker)
+    where, properties = client._type_search_args("application/pdf<&>")
+    # The injected metacharacters are escaped inside the <d:literal>, so they
+    # can't break the SEARCH XML or introduce new elements.
+    assert "pdf&lt;&amp;&gt;" in where
+    assert "pdf<&>" not in where
+    assert "fileid" in properties
+
+
 def test_build_search_xml_emits_offset_only_when_set(mocker):
     client = _make_client(mocker)