feat(mcp): expose parent_id in update_node tool

- Add parent_id to NodeUpdate schema in backend so PATCH /nodes/{id}
  accepts it (was silently ignored before)
- Expose parent_id in update_node MCP tool schema so the MCP SDK
  forwards it to the backend instead of stripping it
- Add regression tests in both backend and MCP layers

mcp/tests/test_tools.py

@@ -26,6 +26,12 @@ async def test_update_node(mock_backend):
     mock_backend.patch.assert_called_once_with("/api/v1/nodes/42", {"label": "New name"})
 
 
+@pytest.mark.anyio
+async def test_update_node_parent_id(mock_backend):
+    await _dispatch("update_node", {"id": "42", "parent_id": "proxmox-1"})
+    mock_backend.patch.assert_called_once_with("/api/v1/nodes/42", {"parent_id": "proxmox-1"})
+
+
 @pytest.mark.anyio
 async def test_delete_node(mock_backend):
     await _dispatch("delete_node", {"id": "42"})