fix(webdav): address PR #764 review round 2

Addresses the four points raised in the automated review on PR #764:

1. Scope guards on the four search tools (search_files, find_by_name,
   find_by_type, list_favorites) so an excluded `scope` raises ToolError
   instead of silently returning an empty result. Previously an LLM
   could probe the asymmetry between list_directory (raises) and the
   search tools (silent) to infer that an excluded directory exists.
   The 4 search tools now mirror the early-guard pattern from
   list_directory and avoid an unnecessary upstream query for known-
   excluded scopes.

2. Concurrent per-tag resolution in get_excluded_file_paths via
   anyio.create_task_group(). Previously the 2N network calls (1
   PROPFIND + 1 REPORT per tag) ran serially. Per-tag fail-open
   behaviour is preserved by extracting _resolve_one_tag, which
   swallows its own exceptions so a single tag failure does not abort
   the surrounding task group.

3. WebDAVClient.get_tag_by_name and get_files_by_tag now route through
   _make_request, inheriting the @retry_on_429 decorator. Previously
   they bypassed it; with tag exclusion invoked on every WebDAV tool
   call, a transient 429 from the systemtags endpoint was hitting the
   fail-open path instead of being transparently retried.

4. Test coverage: 6 new tests in test_webdav_tools_exclusion.py (4
   scope-guard, 2 missing filter tests for find_by_type and
   list_favorites) and 2 new tests in test_tag_exclusion.py (a
   concurrency proof using an event-barrier that would deadlock under
   sequential execution, and a fail-open-under-task-group test with
   order-independent side_effect callables).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

nextcloud_mcp_server/client/webdav.py

@@ -1132,13 +1132,12 @@ class WebDAVClient(BaseNextcloudClient):
   </d:prop>
 </d:propfind>"""
 
-        response = await self._client.request(
+        response = await self._make_request(
             "PROPFIND",
             "/remote.php/dav/systemtags/",
             headers={"Depth": "1"},
             content=propfind_body,
         )
-        response.raise_for_status()
 
         # Parse XML response
         root = ET.fromstring(response.content)
@@ -1216,12 +1215,11 @@ class WebDAVClient(BaseNextcloudClient):
   </oc:filter-rules>
 </oc:filter-files>"""
 
-        response = await self._client.request(
+        response = await self._make_request(
             "REPORT",
             f"{self._get_webdav_base_path()}/",
             content=report_body,
         )
-        response.raise_for_status()
 
         # Parse XML response
         root = ET.fromstring(response.content)