tests: add unit tests for notebook execution CLI

[anish749]

Problem

No tests existed for the notebook execution feature (_exec_notebook, --from-cell/--to-cell, etc.), making it hard to verify behavior before adding the upcoming --output flag.

Solution

Add 8 unit tests covering the notebook execution CLI path:

• Basic multi-cell execution
• Markdown/empty cell filtering
• Fail-fast on cell error
• Progress marker output
• --from-cell / --to-cell range selection
• No-code-cells graceful exit
• Cell-range flags rejected for non-notebook files
• Multi-line output capture

All tests mock send_to_daemon so no running daemon or Jupyter server is needed.