docs(deck): document assignedUsers preservation on cross-board move

Round-6 review: the docstrings listed preserved fields but omitted
assignedUsers. Verified empirically (Deck 1.15.9) that the update route's
board-change handling only remaps labels and leaves user assignments
untouched, so assignees carry over. Documented in both the client and MCP
tool docstrings, with the caveat that an assignee lacking access to the
target board stays assigned but cannot act on the card. Added
test_move_card_to_board_preserves_assigned_users to lock it in.

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

nextcloud_mcp_server/server/deck.py

@@ -1269,10 +1269,12 @@ def configure_deck_tools(mcp: FastMCP):
         """Move a Nextcloud Deck card to a stack on a different board.
 
         The card keeps its identity (same id, comments, attachments), along
-        with its archived state and due date. Deck remaps the card's
-        board-scoped labels to the destination board by title — reusing a
-        same-titled label there, or cloning it when you have board-manage
-        permission. Use deck_reorder_card for moves within a single board.
+        with its archived state, due date and user assignments (an assignee
+        without access to the target board stays assigned but cannot act on the
+        card). Deck remaps the card's board-scoped labels to the destination
+        board by title — reusing a same-titled label there, or cloning it when
+        you have board-manage permission. Use deck_reorder_card for moves
+        within a single board.
 
         Two caveats from Deck's move route: the card's owner is reassigned to
         the user performing the move (the original owner is not preserved), and