fix(review): guard placeholder in scanner reconcile + test dual-write path

Round-1 review follow-ups (PR #857):
- scanner.py: skip the rename-reconcile when the existing metadata point is a
  placeholder. reconcile_document_path only touches real chunks, so a not-yet-
  indexed file would just incur a 0-point set_payload; the real index writes the
  current path anyway.
- test_sharing_state.py: add a dedup-hit case where the file was renamed AND the
  user is new to the ACL, asserting both set_payload writes fire (file_path/title
  and acl_principals).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

nextcloud_mcp_server/vector/scanner.py

@@ -587,13 +587,18 @@ async def scan_user_documents(
                             )
                         )
                         file_queued += 1
-                    elif existing_metadata is not None:
+                    elif existing_metadata is not None and not existing_metadata.get(
+                        "is_placeholder", False
+                    ):
                         # Unchanged content (not re-queued) but the file may have
                         # been renamed/moved: a rename keeps the fileid while
                         # changing the path, and the dedup miss here means the
                         # etag changed without a modified_at bump. Refresh the
                         # stale path/title metadata without re-embedding. No-op
-                        # when the path is unchanged.
+                        # when the path is unchanged. Skip placeholders: reconcile
+                        # only touches real chunks, so a not-yet-indexed file would
+                        # just incur a 0-point set_payload (the real index writes
+                        # the current path anyway).
                         try:
                             await reconcile_document_path(
                                 file_id,