Function create_notebook 

pub async fn create_notebook(
    db: &Arc<dyn NotebookDb>,
    user_id: Uuid,
    name: String,
    cells: Value,
    _deletable: bool,
) -> Result<Notebook, NotebookError>

Create a new notebook.

deletable is always forced to true regardless of the parameter value — this replicates Python’s deletable=deletable or True truthiness bug so the HTTP surface is byte-identical to the Python SDK.