test: automated notebook testing

[planetf1]

Context

Part of the Testing Infrastructure & Strategy Overhaul epic (item 9).

Objective

Evaluate and implement automated notebook testing to catch rot. nbmake is already installed but unused.

Work

• Evaluate nbmake for automated notebook testing (add automated testing for jupyter notebooks #89)
• Implement notebook execution tests — ensure notebooks execute cleanly
• Ensure notebooks are excluded from code coverage metrics
• Define which notebooks should run in CI vs. nightlies (based on backend requirements)
• Update contributor documentation on notebook testing

Notes

• add automated testing for jupyter notebooks #89 is a sub-issue of this work item
• Notebooks should be tested but excluded from coverage (per agreed principles)