fix: avoid nil cursor on callback errors

[nickvanw]

Summary

• return the current cursor instead of nil when delete serialization callbacks fail
• return the current cursor instead of nil when update serialization callbacks fail
• add regression coverage that these callback failures do not panic in Read

Testing

• GOCACHE=/tmp/fivetran-go-build go test ./lib (fails before tests run: link: vitess.io/vitess/go/hack: invalid reference to runtime.roundupsize)

[nickvanw]

Closing in favor of #89, which folds this cursor-safety fix into a single non-stacked PR with the read-error propagation and VStream schema guidance.