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.

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