Refactor how DefaultRowReader determines when to call OperationContext::PostCall

It would be nice to remove the recently added `bool called_post_call_;` member variable and instead refactor how the internal state machine works to ensure OperationContext::PostCall is only called once regardless of success, error, or cancellation.

Locate DefaultRowReader and inspect how its called_post_call_ member coordinates OperationContext::PostCall. Trace the success, error, and cancellation state transitions before deciding how they can guarantee a single PostCall invocation without that member. Done means all three paths call PostCall exactly once, but the issue names no files or tests to run.

Written by the indexing model from the issue text.