fix(webdav): address PR #764 review round 4

- Guard against malformed PROPFIND responses where tag["id"] is None
  before calling get_files_by_tag (prevents
  <oc:systemtag>None</oc:systemtag> dispatch).
- Add OCS-APIRequest: true header to get_tag_by_name and
  get_files_by_tag to match every other PROPFIND/REPORT in the file —
  fixes a latent reverse-proxy compatibility hazard.
- Add test_copy_resource_blocks_excluded_source to mirror the
  existing move-source coverage; closes the asymmetric test gap.
- Add test_skips_tag_with_missing_id covering the new fail-open
  branch in _resolve_one_tag.
- Reword _resolve_one_tag docstring: "distinct slot" was misleading
  (tasks append rather than pre-allocate).

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

nextcloud_mcp_server/client/webdav.py

@@ -1135,7 +1135,11 @@ class WebDAVClient(BaseNextcloudClient):
         response = await self._make_request(
             "PROPFIND",
             "/remote.php/dav/systemtags/",
-            headers={"Depth": "1", "Content-Type": "text/xml"},
+            headers={
+                "Depth": "1",
+                "Content-Type": "text/xml",
+                "OCS-APIRequest": "true",
+            },
             content=propfind_body,
         )
         # Redundant after _make_request (which raises on non-2xx) but
@@ -1223,7 +1227,7 @@ class WebDAVClient(BaseNextcloudClient):
         response = await self._make_request(
             "REPORT",
             f"{self._get_webdav_base_path()}/",
-            headers={"Content-Type": "text/xml"},
+            headers={"Content-Type": "text/xml", "OCS-APIRequest": "true"},
             content=report_body,
         )
         # Redundant after _make_request (which raises on non-2xx) but