Delete outputs on disk if outputs have been cleared

[ellisonbg]

Summary

• Delete output files from disk when notebook cell outputs are cleared in memory
• Pass file_id to process_saving_notebook to enable proper cleanup

Changes

• Modified process_saving_notebook method in OutputsManager to accept file_id parameter
• Added logic to call clear() method when cell outputs are already empty, ensuring disk cleanup
• Updated YRoomFileAPI to pass file_id when processing notebooks for saving

This ensures that when outputs are cleared in the notebook editor, the corresponding output files are also removed from disk, preventing orphaned files and maintaining consistency between memory and disk state.

This is the first part of fixing #134

[3coins]

@ellisonbg
Thanks for making this change, code looks good, left some minor comments.