Add cell execution tracking during notebook execution

[agupta01]

References

Resolves #586

Code changes

This PR adds a completed_cells field to the Job model to track the number of cells executed during notebook execution in real-time. The implementation includes:

Model Changes:

• Added nullable completed_cells column to the Job table in ORM (jupyter_scheduler/orm.py)
• Added completed_cells field to DescribeJob and UpdateJob models (jupyter_scheduler/models.py)

Execution Tracking:

• Added JobFeature.track_cell_execution feature flag to enable/disable cell tracking
• Modified DefaultExecutionManager to use nbconvert's native on_cell_executed hook. Note that this was found to be cleaner than the subclassing approach defined in Track completed cell progress during notebook execution #586.
• Hook updates the database with ep.code_cells_executed after each cell execution

User-facing changes

• GET /jobs/{job_id} now returns the current count of completed cells
• PATCH /jobs/{job_id} accepts completed_cells updates for manual corrections

Backwards-incompatible changes

None

[andrii-i]

Great work @agupta01, thank you. This PR adds cell execution tracking feature in a high-quality and clean manner, db schema changes are done properly and therefore backwards-compatible.

Refactors of validate() and supported_features() to be classmethods makes sense to me and is a clear improvement / cleaner design since both methods don't need instance state.

Two concerns prevent immediate merge:

1. Enabling cell tracking by default makes every cell execution trigger a database write. This could impact production deployments, especially for users with large notebooks and limited compute and networking resources / configurations. Making this feature opt-in / disabled by default would remedy this.
2. Classmethod refactor breaks backwards compatibility for any custom ExecutionManager implementations and therefore is a breaking change. Refactoring these changes into a separate PR that would be merged after the final 2.x release would remedy this.

Happy to merge once these changes are made. The cell tracking feature will be a great addition to Jupyter Scheduler.