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>

nextcloud_mcp_server/client/webdav.py

@@ -905,8 +905,24 @@ class WebDAVClient(BaseNextcloudClient):
             </d:like>
         """
 
+        # fileid is required by callers like NextcloudClient.find_files_by_tag
+        # that dedupe results by id; the default property set in search_files
+        # omits it.
+        properties = [
+            "displayname",
+            "getcontentlength",
+            "getcontenttype",
+            "getlastmodified",
+            "resourcetype",
+            "getetag",
+            "fileid",
+        ]
+
         return await self.search_files(
-            scope=scope, where_conditions=where_conditions, limit=limit
+            scope=scope,
+            where_conditions=where_conditions,
+            properties=properties,
+            limit=limit,
         )
 
     async def list_favorites(