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/server/tag_exclusion.py

@@ -46,11 +46,11 @@ async def _resolve_one_tag(
 ) -> None:
     """Resolve a single tag's paths and append them as a set to *results*.
 
-    Each task writes to a distinct slot in the shared list — append from
-    cooperative tasks is safe under anyio (single-threaded between
-    awaits) without an explicit lock. Swallows its own exceptions so a
-    failure for one tag does not abort the surrounding task group
-    (preserves fail-open per-tag semantics).
+    Each task appends its own set to the shared list; ``list.append`` is
+    atomic between cooperative yields under anyio (single-threaded
+    between awaits) so no explicit lock is needed. Swallows its own
+    exceptions so a failure for one tag does not abort the surrounding
+    task group (preserves fail-open per-tag semantics).
     """
     try:
         tag = await webdav.get_tag_by_name(tag_name)
@@ -67,6 +67,15 @@ async def _resolve_one_tag(
         logger.debug("Excluded tag %r does not exist — skipping", tag_name)
         return
 
+    if tag.get("id") is None:
+        # Malformed PROPFIND response: <oc:systemtag> entry without
+        # <oc:id/>. Skip rather than dispatch <oc:systemtag>None</oc:systemtag>.
+        logger.debug(
+            "Excluded tag %r has no id in PROPFIND response — skipping",
+            tag_name,
+        )
+        return
+
     try:
         files = await webdav.get_files_by_tag(tag["id"])
     except Exception as e: