Create a new notebook or text file, failing if the path already exists.

Unlike /notebook/save (an upsert), this endpoint refuses to overwrite an existing file and returns 409 Conflict when path is taken.

For .ipynb files this writes a starter notebook containing one empty code cell whose cell_id is minted on the backend. That avoids the frontend (useNotebookLoader.makeEmptyCell) fabricating a phantom cell with a client-side id when it loads a zero-cell notebook.