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

backend/app/schemas/nodes.py

@@ -42,6 +42,7 @@ class NodeUpdate(BaseModel):
     notes: str | None = None
     pos_x: float | None = None
     pos_y: float | None = None
+    parent_id: str | None = None
     container_mode: bool | None = None
     custom_colors: dict[str, Any] | None = None
     custom_icon: str | None = None